Fixing queuing bug in outbound flow control.

Motivation:

Outbound flow control does not properly remove the head of queue after
it's written. This will cause streams with multiple frames to get stuck
and not send all of the data.

Modifications:

Modified the DefaultHttp2OutboundFlowController to properly remove the
head of the pending write queue once a queued frame has been written.

Added an integration test that sends a large message to verify that all
DATA frames are properly collected at the other end.

Result:

Outbound flow control properly handles several queued messages.
Handler Unit Test Bug

Motivation:
There is a bug in the JettySslEngineTest where the interface receiving a message does not do a latch.countDown().
This causes each test to be subject to the CountDownLatch timeout period instead of being notified right when an event occurs.

Modifications:
- The JettySslEngineTest message handler will call the appropriate latch.countDown after a message is received

Result:
JettySslEngineTest will not be subject to waiting the timeout period even if the message is correctly received
[#2867] Workaround performance issue with IPv4-mapped-on-IPv6 addresses

Motivation:

InetAddress.getByName(...) uses exceptions for control flow when try to parse IPv4-mapped-on-IPv6 addresses. This is quite expensive.

Modifications:

Detect IPv4-mapped-on-IPv6 addresses in the JNI level and convert to IPv4 addresses before pass to InetAddress.getByName(...) (via InetSocketAddress constructor).

Result:

Eliminate performance problem causes by exception creation when parsing IPv4-mapped-on-IPv6 addresses.
Allow to write CompositeByteBuf directly via EpollDatagramChannel. Related to [#2719]

Motivation:

On linux it is possible to use the sendMsg(...) system call to write multiple buffers with one system call when using datagram/udp.

Modifications:

- Implement the needed changes and make use of sendMsg(...) if possible for max performance
- Add tests that test sending datagram packets with all kind of different ByteBuf implementations.

Result:

Performance improvement when using CompoisteByteBuf and EpollDatagramChannel.
Add support for sendmmsg(...) and so allow to write multiple DatagramPackets with one syscall. Related to [#2719]

Motivation:

On linux with glibc >= 2.14 it is possible to send multiple DatagramPackets with one syscall. This can be a huge performance win and so we should support it in our native transport.

Modification:

- Add support for sendmmsg by reuse IovArray
- Factor out ThreadLocal support of IovArray to IovArrayThreadLocal for better separation as we use IovArray also without ThreadLocal in NativeDatagramPacketArray now
- Introduce NativeDatagramPacketArray which is used for sendmmsg(...)
- Implement sendmmsg(...) via jni
- Expand DatagramUnicastTest to test also sendmmsg(...)

Result:

Netty now automatically use sendmmsg(...) if it is supported and we have more then 1 DatagramPacket in the ChannelOutboundBuffer and flush() is called.
Fix buffer leak in DatagramUnicastTest caused by incorrect usage of CompositeByteBuf

Motivation:

Due incorrect usage of CompositeByteBuf a buffer leak was introduced.

Modifications:

Correctly handle tests with CompositeByteBuf.

Result:

No more buffer leaks
Directly write CompositeByteBuf if possible without memory copy. Related to [#2719]

Motivation:

In linux it is possible to write more then one buffer withone syscall when sending datagram messages.

Modifications:

Not copy CompositeByteBuf if it only contains direct buffers.

Result:

More performance due less overhead for copy.
HTTP Content Decoder Cleanup Bug

Motiviation:
The HTTP content decoder's cleanup method is not cleaning up the decoder correctly.
The cleanup method is currently doing a readOutbound on the EmbeddedChannel but
for decoding the call should be readInbound.

Modifications:
-Change readOutbound to readInbound in the cleanup method

Result:
The cleanup method should be correctly releaseing unused resources
Correct javadoc typo in IdleStateHandler example code
HTTP/2 Cipher Suite Support

Motivation:
The HTTP/2 specification places restrictions on the cipher suites that can be used. There is no central place to pull the ciphers that are allowed by the specification, supported by different java versions, and recommended by the community.

Modifications:
-HTTP/2 will have a security utility class to define supported ciphers
-netty-handler will be modified to support filtering the supplied list of ciphers to the supported ciphers for the current SSLEngine

Result:
-Netty provides unified support for HTTP/2 cipher lists and ciphers can be pruned by currently supported ciphers