Access autoRead via an AtomicIntegerFieldUpdater.

Motiviation:

Before this change, autoRead was a volatile boolean accessed directly.  Any thread that invoked the DefaultChannelConfig#setAutoRead(boolean) method would read the current value of autoRead, and then set a new value.  If the old value did not match the new value, some action would be immediately taken as part of the same method call.

As volatile only provides happens-before consistency, there was no guarantee that the calling thread was actually the thread mutating the state of the autoRead variable (such that it should be the one to invoke the follow-up actions).  For example, with 3 threads:
 * Thread 1: get = false
 * Thread 1: set = true
 * Thread 1: invokes read()
 * Thread 2: get = true
 * Thread 3: get = true
 * Thread 2: set = false
 * Thread 2: invokes autoReadCleared()
 * Event Loop receives notification from the Selector that data is available, but as autoRead has been cleared, cancels the operation and removes read interest
 * Thread 3: set = true

This results in a livelock - autoRead is set true, but no reads will happen even if data is available (as readyOps).  The only way around this livelock currently is to set autoRead to false, and then back to true.

Modifications:

Write access to the autoRead variable is now made using the getAndSet() method of an AtomicIntegerFieldUpdater, AUTOREAD_UPDATER.  This also changed the type of the underlying autoRead variable to be an integer, as no AtomicBooleanFieldUpdater class exists.  Boolean logic is retained by assuming that 1 is true and 0 is false.

Result:

There is no longer a race condition between retrieving the old value of the autoRead variable and setting a new value.
Add the encoder/decoder getter methods to HttpClientCodec

Motivation:

There's no way for a user to get the encoder and the decoder of an
HttpClientCodec.  The lack of such getter methods makes it impossible to
remove the codec handlers from the pipeline correctly.

For example, a user could add more than one HttpClientCodec to the
pipeline, and then the user cannot easily decide which encoder and
decoder to remove.

Modifications:

- Add encoder() and decoder() method to HttpClientCodec which returns
  HttpRequestEncoder and HttpResponseDecoder respectively
- Also made the same changes to HttpServerCodec

Result:

A user can distinguish the handlers added by multiple HttpClientCodecs
easily.
Auto-generate the handler name when null is specified as a name

Motivation:

There's no way to generate the name of a handler being newly added
automatically and reliably.

For example, let's say you have a routine that adds a set of handlers to
a pipeline using addBefore() or addAfter().  Because addBefore() and
addAfter() always require non-conflicting non-null handler name, making
the multiple invocation of the routine on the same pipeline is
non-trivial.

Modifications:

- If a user specifies null as the name of the new handler,
  DefaultChannelPipeline generates one.
- Update the documentation of ChannelPipeline to match the new behavior

Result:

A user doesn't need to worry about name conflicts anymore.
Fix a bug in NetUtil.createByteArrayFromIpAddressString()

Motivation:

An IPv6 string can have a zone index which is followed by the '%' sign.
When a user passes an IPv6 string with a zone index,
NetUtil.createByteArrayFromIpAddressString() returns an incorrect value.

Modification:

- Strip the zone index before conversion

Result:

An IPv6 string with a zone index is decoded correctly.
Fix an incorrect use of ByteBuf.array() in Socks5CmdRequestDecoder

Motivation:

Socks5CmdRequestDecoder uses ByteBuf.array() naively assuming that the
array's base offset is always 0, which is not the case.

Modification:

- Allocate a new byte array and copy the content there instead

Result:

Another bug fixed
Add AbstractUnsafe.annotateConnectException()

Motivation:

JDK's exception messages triggered by a connection attempt failure do
not contain the related remote address in its message.  We currently
append the remote address to ConnectException's message, but I found
that we need to cover more exception types such as SocketException.

Modifications:

- Add AbstractUnsafe.annotateConnectException() to de-duplicate the
  code that appends the remote address

Result:

- Less duplication
- A transport implementor can annotate connection attempt failure
  message more easily
Add proxy support for client socket connections

Related issue: #1133

Motivation:

There is no support for client socket connections via a proxy server in
Netty.

Modifications:

- Add a new module 'handler-proxy'
- Add ProxyHandler and its subclasses to support SOCKS 4a/5 and HTTP(S)
  proxy connections
- Add a full parameterized test for most scenarios
- Clean up pom.xml

Result:

A user can make an outgoing connection via proxy servers with only
trivial effort.
Do not consider PortUnreachableException to require channel closure

Motivation:

When a datagram packet is sent to a destination where nobody actually listens to,
the server O/S will respond with an ICMP Port Unreachable packet.
The ICMP Port Unreachable packet is translated into PortUnreachableException by JDK.
PortUnreachableException is not a harmful exception that prevents a user from sending a datagram.
Therefore, we should not close a datagram channel when PortUnreachableException is caught.

Modifications:

- Do not close a channel when the caught exception is PortUnreachableException.

Result:

A datagram channel is not closed unexpectedly anymore.
Add EDNS support to DnsQueryEncoder

Motivation:

DnsQueryEncoder does not encode the 'additional resources' section at all, which contains the pseudo-RR as defined in RFC 2671.

Modifications:

- Modify DnsQueryEncoder to encode the additional resources
- Fix a bug in DnsQueryEncoder where an empty name is encoded incorrectly

Result:

A user can send an EDNS query.
Name resolver API and DNS-based name resolver

Motivation:

So far, we relied on the domain name resolution mechanism provided by
JDK.  It served its purpose very well, but had the following
shortcomings:

- Domain name resolution is performed in a blocking manner.
  This becomes a problem when a user has to connect to thousands of
  different hosts. e.g. web crawlers
- It is impossible to employ an alternative cache/retry policy.
  e.g. lower/upper bound in TTL, round-robin
- It is impossible to employ an alternative name resolution mechanism.
  e.g. Zookeeper-based name resolver

Modification:

- Add the resolver API in the new module: netty-resolver
- Implement the DNS-based resolver: netty-resolver-dns
  .. which uses netty-codec-dns
- Make ChannelFactory reusable because it's now used by
  io.netty.bootstrap, io.netty.resolver.dns, and potentially by other
  modules in the future
  - Move ChannelFactory from io.netty.bootstrap to io.netty.channel
  - Deprecate the old ChannelFactory
  - Add ReflectiveChannelFactory

Result:

It is trivial to resolve a large number of domain names asynchronously.