Upgrading to HTTP/2 draft 14 framing

Motivation:

HTTP/2 draft 14 came out a couple of weeks ago and we need to keep up
with the spec.

Modifications:

- Removed use of segment throughout.
- Added new setting for MAX_FRAME_SIZE. Used by the frame reader/writer
rather than a constant.
- Added new setting for MAX_HEADER_LIST_SIZE. This is currently unused.
- Expanded the header size to 9 bytes. The frame length field is now 3
bytes and added logic for checking that it falls within the valid range.

Result:

Netty will support HTTP/2 draft 14 framing. There will still be some
work to do to be compliant with the HTTP adaptation layer.
Add HTTP/2 codec to netty-all maven dependency Motivation:

The HTTP/2 codec is missing from the netty-all maven dependency.
This means third party libraries have to pull in additional dependencies
above netty-all to use this codec.

Modifications:

Add HTTP/2 codec to netty-all

Result:

Third party libraries don't need to pull in additional dependencies
to use HTTP/2 codec.
codec-socks test cleanup

Motivation:

The unit tests in codec-socks contained redundant casts and empty test classes.

Modifications:

- Remove redundant casts
- Delete empty test classes

Result:

Cleanup
[#2586] Use correct EventExecutor to notify for bind failures on late registration

Motivation:

We used the wrong EventExecutor to notify for bind failures if a late registration was done.

Modifications:

Use the correct EventExecutor to notify and only use the GlobelEventExecutor if the registration fails itself.

Result:

The correct Thread will do the notification.
[#2800] Fix NPE in SingleThreadEventExecutor on Android

Motivation:

The recent changes f8bee2e94c33f167bcdefe3e8bbf95e99765032e to use a FJP introduced a regression on Android that cause a NPE.

Modifications:

- Use AtomicReferenceFieldUpdater so it works also on Android
- Fix inspection warnings

Result:

Netty works again on Android too.
Code-inspection fixes

Motivation:

Saw some code-inspection warnings

Modifications:

Fix warnings

Result:

Less code-inspection warnings
Avoid redundant reads of head in peakNode

Motivation:

There is not need todo redunant reads of head in peakNode as we can just spin on next() until it becomes visible.

Modifications:

Remove redundant reads of head in peakNode. This is based on @nitsanw's patch for akka.
See https://github.com/akka/akka/pull/15596

Result:

Less volatile access.
Allow efficient writing of CompositeByteBuf when using native epoll transport.

Motivation:

There were no way to efficient write a CompositeByteBuf as we always did a memory copy to a direct buffer in this case. This is not needed as we can just write a CompositeByteBuf as long as all the components are buffers with a memory address.

Modifications:

- Write CompositeByteBuf which contains only direct buffers without memory copy
- Also handle CompositeByteBuf that have more components then 1024.

Result:

More efficient writing of CompositeByteBuf.
Reduce memory copies in spdy compression implementation.

Motivation:

Currently we do more memory copies then needed.

Modification:

- Directly use heap buffers to reduce memory copy
- Correctly release buffers to fix buffer leak

Result:

Less memory copies and no leaks
Pluggable algorithm to choose next EventLoop
Motivation:

Currently when a new event loop is needed to register a channel our EventLoopGroup implementations just
use a round-robin like algorithm to choose the next event loop. Unfortunately this is not always good
enough as different event loops may become more busy than others over time. This is especially true when
an application handles different kinds of connections (long-living and short-living). This change allows
developers to plug in their own algorithms to choose the next event loop and to collect metrics about the
busyness of the event loops. See #1230

Modifications:

- TODO

Result:

- Developers can plug in their own algorithms into MultithreadEventLoopGroup constructors.
- Developers can gather metrics/feedback from the event loops and use the data in their custom algorithms.