[#2924] Correctly update head in MemoryRegionCache.trim()

Motivation:
When MemoryRegionCache.trim() is called, some unused cache entries will be freed (started from head). However, in MeoryRegionCache.trim() the head is not updated, which make entry list's head point to an entry whose chunk is null now and following allocate of MeoryRegionCache will return false immediately.

In other word, cache is no longer usable once trim happen.

Modifications:

Update head to correct idx after free entries in trim().

Result:

MemoryRegionCache behaves correctly even after calling trim().
Disable caching of PooledByteBuf for different threads.

Motivation:

We introduced a PoolThreadCache which is used in our PooledByteBufAllocator to reduce the synchronization overhead on PoolArenas when allocate / deallocate PooledByteBuf instances. This cache is used for both the allocation path and deallocation path by:
  - Look for cached memory in the PoolThreadCache for the Thread that tries to allocate a new PooledByteBuf and if one is found return it.
  - Add the memory that is used by a PooledByteBuf to the PoolThreadCache of the Thread that release the PooledByteBuf

This works out very well when all allocation / deallocation is done in the EventLoop as the EventLoop will be used for read and write. On the otherside this can lead to surprising side-effects if the user allocate from outside the EventLoop and and pass the ByteBuf over for writing. The problem here is that the memory will be added to the PoolThreadCache that did the actual write on the underlying transport and not on the Thread that previously allocated the buffer.

Modifications:

Don't cache if different Threads are used for allocating/deallocating

Result:

Less confusing behavior for users that allocate PooledByteBufs from outside the EventLoop.
[#2926] Fix 1 byte memory leak in native transport

Motivation:

We use malloc(1) in the on JNI_OnLoad method but never free the allocated memory. This means we have a tiny memory leak of 1 byte.

Modifications:

Call free(...) on previous allocated memory.

Result:

Fix memory leak
IPv6 address to string rfc5952

Motivation:
The java implementations for Inet6Address.getHostName() do not follow the RFC 5952 (http://tools.ietf.org/html/rfc5952#section-4) for recommended string representation. This introduces inconsistencies when integrating with other technologies that do follow the RFC.

Modifications:
-NetUtil.java to have another public static method to convert InetAddress to string. Inet4Address will use the java InetAddress.getHostAddress() implementation and there will be new code to implement the RFC 5952 IPV6 string conversion.
-New unit tests to test the new method

Result:
Netty provides a RFC 5952 compliant string conversion method for IPV6 addresses
HTTP/2 Read Decompression Flow Control Fix

Motivation:
The current implementation of the HTTP/2 decompression does not integrate with flow control properly.
The decompression code is giving the post-decompression size to the flow control algorithm which
results in flow control errors at incorrect times.

Modifications:
-DecompressorHttp2FrameReader.java will need to change where it hooks into the HTTP/2 codec
-Enhance unit tests to test this condition

Result:
No more flow control errors because of decompression design flaw
Updating HTTP/2 tests to not retain buffers for validation

Motivation:

To eliminate the tests as being a cause of leaks, removing the automatic
retaining of ByteBufs in Http2TestUtil.

Modifications:

Each test that relied on retaining buffers for validation has been
modified to copy the buffer into a list of Strings that are manually
validated after the message is received.

Result:

The HTTP/2 tests should (hopefully) no longer be reporting erroneous
leaks due to the testing code, itself.
HTTP/2 to HTTP buffer ordering

Motivation:
The HTTP tranlsation layer uses a FullHttpMessage object after it has been fired up the pipeline.
Although the content ByteBuf is not used by default it is still not ideal to use a releasable object
after it has potentially been released.

Modifications:
-InboundHttp2ToHttpAdapter ordering issues will be corrected

Result:
Safer access to releasable objects in the HTTP/2 to HTTP translation layer.
[#2939] Fix SslContext usage in the examples for client side

Motivation:

We incorrectly used SslContext.newServerContext() in some places where a we needed a client context.

Modifications:

Use SslContext.newClientContext() when using ssl on the client side.

Result:

Working ssl client examples.
HTTP/2 Codec Read/Write Restructure

Motivation:
The HTTP/2 codec has some duplication and the read/write interfaces are not cleanly exposed to users of the codec.

Modifications:
-Restructure the AbstractHttp2ConnectionHandler class to be able to extend write behavior before the outbound flow control gets the data
-Add Http2InboundConnectionHandler and Http2OutboundConnectionHandler interfaces and restructure external codec interface around these concepts

Result:
HTTP/2 codec provides a cleaner external interface which is easy to extend for read/write events.
Send a websocket close frame with status code when receiving invalid frames

Motivation:

According to the websocket specification peers may send a close frame when
they detect a protocol violation (with status code 1002). The current
implementation simply closes the connection. This update should add this
functionality. The functionality is optional - but it might help other
implementations with debugging when they receive such a frame.

Modification:

When a protocol violation in the decoder is detected and a close was not
already initiated by the remote peer a close frame is
sent.

Result:

Remotes which will send an invalid frame will now get a close frame that
indicates the protocol violation instead of only seeing a closed
connection.
HTTP/2 Unit Tests Synchronized Collections

Motivation:
The HTTP/2 unit tests are collecting responses read events which are happening in a multithreaded environment.
These collections are currently not synchronized or thread safe and are resulting in verification failures.

Modifications:
-Modify unit tests that use collections to store results for verifiction to be thread safe

Result:
Tests should not fail because of syncrhonization issues while verifying expected results.
Netty Headers Class Restructure and Algorithm Updates

Motivation:
Headers within netty do not cleanly share a common class hierarchy.  As a result some header types support some operations
and don't support others.  The consolidation of the class hierarchy will allow for maintenance and scalability for new codec.
The existing hierarchy also has a few short comings such as it is not clear when data conversions are happening.  This
could result unintentionally getting back a collection or iterator where a conversion on each entry must happen.

The current headers algorithm also prepends all elements which means to find the first element or return a collection
in insertion order often requires a complete traversal followed by a collections.reverse call.

Modifications:
-Provide a generic base class which provides all the implementation for headers in netty
-Provide an extension to this class which allows for name type conversions to happen (to accommodate legacy CharSequence to String conversions)
-Update the headers interface to clarify when conversions will happen.
-Update the headers data structure so that appends are done to avoid unnecessary iteration or collection reversal.

Result:
-More unified class hierarchy for headers in netty
-Improved headers data structure and algorithms
-headers API more clearly identify when conversions are required.