Fixing HTTP/2 data write completion bug

Motivation:

A recent refactoring of the outbound flow controller interface
introduced a bug when writing data. We're no longer properly handling
the completion of the write (i.e. updating stream state/handling error).

Modifications:

Updated AbstractHttp2ConnectionHandler.writeData to properly handle the
completion of the write future.

Result:

DATA writes now perform post-write cleanup.
[#2841] Fix SingleThreadEventLoopTest that was failing because of GC pressure

Motivation:

Sometimes the SingleThreadEventLoopTest did fail on our CI. This was because of GC pressure produced by Thread.sleep(...) when interrupted as it creates a new InterruptedException all the time (and need to fill it).

Modifications:

Replace Thread.sleep(...) with LockSupport.parkNanos(...) to eliminate exception overhead.

Result:

SingleThreadEventLoopTest produce a lot less garbage.
Cleanup test

Motivation:

Saw a lot of inspector warnings

Modifications:

Fix inspector warnings

Result:

Cleaner code
Reduce Memory Usage of SingleThreadEventLoopTest

Motivation:

The SingleThreadEventLoopTest allocated up to half a gigabyte of memory per run,
causing up to 50 GC runs in ~20 seconds.

Modification:

Charlie Hunt identified TLAB allocations to be the root cause of this excessive
memory usage [1]. By reusing the Executor in every test we can reduce the
memory usage by >50%.

Result:

Lower Memory Usage and fewer Garbage Collector runs.
Helps to resolve GitHub issue #2841.

[1] https://twitter.com/charlesjhunt/status/505351389317722112
ALPN java implementation
Motivation:

Netty only supports a java NPN implementation provided by npn-api and npn-boot.
There is no java implementation for ALPN.
ALPN is needed to be compliant with the HTTP/2 spec.

Modifications:
-SslContext and JdkSslContext to support ALPN
-JettyNpn* class restructure for NPN and ALPN common aspects
-Pull in alpn-api and alpn-boot optional dependencies for ALPN java implementation

Result:

-Netty provides access to a java implementation of APLN
Correctly release buffer in DelegatingHttp2ConnectionHandlerTest

Motivation:

Because we did not release the buffer correctly in DelegatingHttp2ConnectionHandlerTest the CI failed because of a buffer leak.
See https://secure.motd.kr/jenkins/job/pr_2849_kichik_netty_master/4/consoleFull

Modifications:

Correctly release buffer

Result:

No more leak error.
Correctly release buffers on protocol errors

Motivation:

We failed to release buffers on protocolErrors which caused buffer leaks when using HTTP/2

Modifications:

Release buffer on protocol errors

Result:

No more buffer leaks
Don't ignore maxCapacity if it's not a power of 2

Motivation:

This fixes bug #2848 which caused Recycler to become unbounded and cache infinite number of objects with maxCapacity that's not a power of two. This can result in general sluggishness of the application and OutOfMemoryError.

Modifications:

The test for maxCapacity has been moved out of test to check if the buffer has filled. The buffer is now also capped at maxCapacity and cannot grow over it as it jumps from one power of two to the other.

Additionally, a unit test was added to verify maxCapacity is honored even when it's not a power of two.

Result:

With these changes the user is able to use a custom maxCapacity number and not have it ignored. The unit test assures this bug will not repeat itself.
[#2847] Correctly encode HTTP to SPDY if X-SPDY-Associated-To-Stream-ID is not present

Motivation:

Because of a bug a NPE was thrown when tried to encode HTTP to SPDY and no X-SPDY-Associated-To-Stream-ID was present.

Modifications:

Use 0 as default value when X-SPDY-Associated-To-Stream-ID not present.

Result:

No NPE anymore.