Grab-bag of minor changes to HTTP/2

Motivation:

Various small fixes/improvements to the interface to the HTTP/2 classes,
as well as some minor performance improvements.

Modifications:

- Added fix for IntObjectHashMap to ensure that capacity is always odd.
Even capacity can cause probing to fail.
- Cleaned the access to GOAWAY information in Http2Connection interface.
Endpoints now manage their own state for GOAWAY. Also added a goingAway
event handler.
- Added Endpoint methods for checking MAX_CONCURRENT_STREAMS or if the
number of streams for the endpoint have been exhausted. See
Endpoint.nextStreamId()/acceptingNewStreams().
- Changed DefaultHttp2Connection to use IntObjectHashMap. This should be
a slight memory improvement.
- Fixed check for MAX_CONCURRENT_STREAMS to correctly use the number of
active streams for the endpoint (not total active). See
DefaultHttp2Connection.checkNewStreamAllowed.
- Exposing a few methods to subclasses of AbstractHttp2ConnectionHandler
(e.g. exception handling).
- Cleaning up GOAWAY and RST_STREAM handling in
AbstractHttp2ConnectionHandler.

Result:

HTTP/2 code should provide more information to subclasses and will have
a reduced memory footprint.
[#2586] Use correct EventLoop to notify delayed successful registration

Motivation:

At the moment AbstractBoostrap.bind(...) will always use the GlobalEventExecutor to notify the returned ChannelFuture if the registration is not done yet. This should only be done if the registration fails later. If it completes successful we should just notify with the EventLoop of the Channel.

Modification:

Use EventLoop of the Channel if possible to use the correct Thread to notify and so guaranteer the right order of events.

Result:

Use the correct EventLoop for notification
[#2580] [#2587] Fix buffer corruption regression when ByteBuf.order(LITTLE_ENDIAN) is used

Motivation:

To improve the speed of ByteBuf with order LITTLE_ENDIAN and where the native order is also LITTLE_ENDIAN (intel) we introduces a new special SwappedByteBuf before in commit 4ad3984c8b725ef59856d174d09d1209d65933fc. Unfortunally the commit has a flaw which does not handle correctly the case when a ByteBuf expands. This was caused because the memoryAddress was cached and never changed again even if the underlying buffer expanded. This can lead to corrupt data or even to SEGFAULT the JVM if you are lucky enough.

Modification:

Always lookup the actual memoryAddress of the wrapped ByteBuf.

Result:

No more data-corruption for ByteBuf with order LITTLE_ENDIAN and no JVM crashes.
Remove System.out.println(...) debug messages
[#2589] LocalServerChannel.doClose() throws NPE when localAddress == null

Motivation:

LocalServerChannel.doClose() calls LocalChannelRegistry.unregister(localAddress); without check if localAddress is null and so produce a NPE when pass null the used ConcurrentHashMapV8

Modification:
Check for localAddress != null before try to remove it from Map. Also added a unit test which showed the stacktrace of the error.

Result:

No more NPE during doClose().
Improve the allocation algorithm in PoolChunk

Motivation:

Depth-first search is not always efficient for buddy allocation.

Modification:

Employ a new faster search algorithm with different memoryMap layout.

Result:

With thread-local cache disabled, we see a lot of performance
improvment, especially when the size of the allocation is as small as
the page size, which had the largest search space previously:

-- master head --
Benchmark                (size) Mode    Score  Error Units
pooledDirectAllocAndFree  8192 thrpt  215.392  1.565 ops/ms
pooledDirectAllocAndFree 16384 thrpt  594.625  2.154 ops/ms
pooledDirectAllocAndFree 65536 thrpt 1221.520 18.965 ops/ms
pooledHeapAllocAndFree    8192 thrpt  217.175  1.653 ops/ms
pooledHeapAllocAndFree   16384 thrpt  587.250 14.827 ops/ms
pooledHeapAllocAndFree   65536 thrpt 1217.023 44.963 ops/ms

-- changes --
Benchmark                (size) Mode    Score  Error Units
pooledDirectAllocAndFree  8192 thrpt 3656.744 94.093 ops/ms
pooledDirectAllocAndFree 16384 thrpt 4087.152 22.921 ops/ms
pooledDirectAllocAndFree 65536 thrpt 4058.814 29.276 ops/ms
pooledHeapAllocAndFree    8192 thrpt 3640.355 44.418 ops/ms
pooledHeapAllocAndFree   16384 thrpt 4030.206 24.365 ops/ms
pooledHeapAllocAndFree   65536 thrpt 4103.991 70.991 ops/ms
Fix the inconsistencies between performance tests in ByteBufAllocatorBenchmark

Motivation:

default*() tests are performing a test in a different way, and they must be same with other tests.

Modification:

Make sure default*() tests are same with the others

Result:

Easier to compare default and non-default allocators
Add HAProxy protocol decoder

Motivation:

The proxy protocol provides client connection information for proxied
network services. Several implementations exist (e.g. Haproxy, Stunnel,
Stud, Postfix), but the primary motivation for this implementation is to
support the proxy protocol feature of Amazon Web Services Elastic Load
Balancing.

Modifications:

This commit adds a proxy protocol decoder for proxy protocol version 1
as specified at:

  http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt

The foundation for version 2 support is also in this commit but it is
explicitly NOT supported due to a lack of external implementations to
test against.

Result:

The proxy protocol decoder can be used to send client connection
information to inbound handlers in a channel pipeline from services
which support the proxy protocol.
Move haproxy codec to a separate module
Overall refactoring of the haproxy codec

- Convert constant classes to enum
- Rename HAProxyProtocolMessage to HAProxyMessage for simpilicity
- Rename HAProxyProtocolDecoder to HAProxyMessageDecoder
- Rename HAProxyProtocolCommand to HAProxyCommand
- Merge ProxiedProtocolAndFamity, ProxiedAddressFamily, and
  ProxiedTransportProtocol into HAProxiProxiedProtocol and its inner
  enums
- Overall clean-up
Checkstyle
Add MQTT protocol codec

MQTT is a open source protocol on top of TCP which is widely used in
mobile communication and also for IoT (Internet of Things) today. This
will add an open source implementation of MQTT so that it becomes easier
for Netty users to implement an MQTT application.

For more information about the MQTT protocol, read this:

http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html
Overall clean-up on codec-mqtt

- Use simple string concatenation instead of String.format()
- Rewrite exception messages so that it follows our style
- Merge MqttCommonUtil and MqttValidationUtil into MqttCodecUtil
- Hide MqttCodecUtil from users
- Rename MqttConnectReturnCode.value to byteValue
- Rename MqttMessageFactory.create*() to new*()
- Rename QoS to MqttQoS
- Make MqttSubAckPayload.grantedQoSLevels immutable and add more useful
  constructor
Rename io.netty.util.collection.Collections to PrimitiveCollections

Motivation:

An IDE's auto-completion often confuses between java.util.Collections
and io.netty.util.collection.Collections, and it's annoying to me. :-)

Modifications:

Use a different class name.

Result:

When I type Collections, it's always 'the' Collections.
Fix test failures due to incorrect validation
Mqtt -> MQTT
Fix false sharing between head and tail reference in MpscLinkedQueue

Motivation:

The tail node reference writes (by producer threads) are very likely to
invalidate the cache line holding the headRef which is read by the
consumer threads in order to access the padded reference to the head
node. This is because the resulting layout for the object is:

- header
- Object AtomicReference.value -> Tail node
- Object MpscLinkedQueue.headRef -> PaddedRef -> Head node

This is 'passive' false sharing where one thread reads and the other
writes.  The current implementation suffers from further passive false
sharing potential from any and all neighbours to the queue object as no
pre/post padding is provided for the class fields.

Modifications:

Fix the memory layout by adding pre-post padding for the head node and
putting the tail node reference in the same object.

Result:

Fixed false sharing
Checkstyle / Overall clean-up / Fix serialization
Add missing last padding / Comment
Remove padding utility classes

- It's not used anywhere