[#2363] SelectedSelectionKeySet may hold strong reference to SelectionKey after Channel is closed

Motivation:
Because we not null out the array entry in the SelectionKey[] which is produced by SelectedSelectionKeySet.flip() we may end up with a few SelectionKeyreferences still hanging around here even after the Channel was closed. As these entries may be present at the end of the SelectionKey[] which is never updated for a long time as not enough SelectionKeys are ready.

Modifications:
Once we access the SelectionKey out of the SelectionKey[] we directly null it out.

Result:
Reference can be GC'ed right away once the Channel was closed.
Removing HTTP2 module's dependency on guava

Motivation:

The use of the guava library library is fairly superficial and can
easily be removed to reduce the overall dependency list of the module.
Cleaning up the HTTP2 code to use common Netty utilities, etc.

Modifications:

- Various changes to remove use of guava preconditions and collections.
- Changed default charset to use CharsetUtil.UTF_8.
- Changed StreamState to use ArrayDeque instead of LinkedList.
- Changed precondition checks to throw NPE
- Changed implementation of Http2Headers to more closely mirror the SPDY
implementation.

Result:

The behavior of the HTTP2 module will remain unchanged.
Next round of cleanup of http2 codec

Motivation:
Bring the http2 codec more inline with the rest of Netty's code-base.

Modifications:

- Add null checks in constructors
- Move private static methods into inner-classes where they are used
- Use getBytes(CharsetUtil.UTf8)

Result:
More consistent code-base
Allow the user to call slice().retain() or duplicate.retain() in his/her ByteToMessageDecoder.decode(...) method.

Motivation:
At the moment a user can not safetly call slice().retain() or duplicate.retain()in the ByteToMessageDecoder.decode(...) implementation without the risk to see coruption because we may call discardSomeReadBytes() to make room on the buffer once the handling is done.

Modifications:
Check for the refCnt() before call discardSomeReadBytes() and also check before call decode(...) to create a copy if needed.

Result:
The user can safetly call slice().retain() or duplicate.retain() in his/her ByteToMessageDecoder.decode(...) method.
Adding examples for HTTP/2 framing.

Motivation:

Provide some example code to show how to bootstrap client and server for
use with HTTP/2 framing.

Modifications:

- Fixed Http2ConnectionHandler to allow headers after stream creation.
Needed for response headers.

- Added toString() to all frame classes to help with debugging/logging

- Added example classes for HTTP/2

Result:

HTTP/2 connections now properly support response headers. Examples for
HTTP/2 provided with the distribution of examples module.

After your change, what will change.
[#2353] Use a privileged block to get ClassLoader and System property if needed

Motivation:
When using System.getProperty(...) and various methods to get a ClassLoader it will fail when a SecurityManager is in place.

Modifications:
Use a priveled block if needed. This work is based in the PR #2353 done by @anilsaldhana .

Result:
Code works also when SecurityManager is present
[#2370] Periodically check for not alive Threads and free up their ThreadPoolCache

Motivation:
At the moment we create new ThreadPoolCache whenever a Thread tries either allocate or release something on the PooledByteBufAllocator. When something is released we put it then in its ThreadPoolCache. The problem is we never check if a Thread is not alive anymore and so we may end up with memory that is never freed again if a user create many short living Threads that use the PooledByteBufAllocator.

Modifications:
Periodically check if the Thread is still alive that has a ThreadPoolCache assinged and if not free it.

Result:
Memory is freed up correctly even for short living Threads.
[#2371] Fix Potential data corruption in EpollSocketChannel when writing to the Channel

Motivation:
We sometimes see data corruption when writing to the EpollSocketChannel.

Modifications:
Correctly update the position of the ByteBuffer after something was written.

Result:
Fix data-corruption which could happen on partial writes
Adding support for HTTP/2 initialization handshake.

Motivation:

The current HTTP/2 support does not properly comply with the HTTP/2 spec
wrt startup.

Modifications:

Changed the frame codec as well as the connection handler to support
exchange of the connection preface, followed immediately by an initial
settings frame.

Result:

The HTTP/2 initialization handshake will be in compliance with the spec.
Will need more work to support the upgrade protocols, however :)
Add a Example for Memcache Binary Codec

Motivation:
Currently, there exists no example which shows how to use the memcache binary
protocol.

Modifications:
Add an example client and client handler to show how to utilize the binary
protocol in a memcache client with a simple interactive shell.

Result:
Users looking for an example can now start off with the provided one.