Adding missing asserts to HTTP/2 tests

Motivation:

Some tests do not properly assert that all requests have been
sent/received, so the failures messages may be misleading.

Modifications:

Adding missing asserts to HTTP/2 tests for awaiting requests and
responses.

Result:

HTTP/2 tests properly assert message counts.
Fix Http/2 example response timeout

Motivation:

The HTTP/2 example can timeout at the client waiting for a response due
to the server not flushing after writing the response.

Modifications:

Updated the server's HelloWorldHttp2Handler to flush after writing the
response.

Result:

The HTTP/2 example runs successfully.
Fixing concurrency issue with HTTP/2 test mocking

Motivation:

Some tests occasionally appear unstable, throwing a
org.mockito.exceptions.misusing.UnfinishedStubbingException. Mockito
stubbing does not work properly in multi-threaded environments, so any
stubbing has to be done before the threads are started.

Modifications:

Modified tests to perform any custom stubbing before the client/server
bootstrap logic executes.

Result:

HTTP/2 tests should be more stable.
Simplifying and centralizing HTTP/2 exception handling logic

Motivation:

Currently, Http2LifecycleManager implements the exception handling logic
which makes it difficult to extend or modify the exception handling
behavior.  Simply overriding exceptionCaught() will only affect one of
the many possible exception paths. We need to reorganize the exception
handling code to centralize the exception handling logic into a single
place that can easily be extended by subclasses of
Http2ConnectionHandler.

Modifications:

Made Http2LifecycleManager an interface, implemented directly by
Http2ConnectionHandler. This adds a circular dependency between the
handler and the encoder/decoder, so I added builders for them that allow
the constructor of Http2ConnectionHandler to set itself as the lifecycle
manager and build them.

Changed Http2LifecycleManager.onHttpException to just
onException(Throwable) to simplify the interface. This method is now the
central control point for all exceptions. Subclasses now only need to
override onException() to intercept any exception encountered by the
handler.

Result:

HTTP/2 has more extensible exception handling, that is less likely to
see exceptions vanish into the ether.
Restoring access to HTTP/2 decoder's listener.

Motivation:

This was lost in recent changes, just adding it back in.

Modifications:

Added listener() accessor to Http2ConnectionDecoder and the default
impl.

Result:

The Http2FrameListener can be obtained from the decoder.
NullPointerException fix in http/2 codec

Motivation:
There is a NPE due to the order of builder initialization in the  class.

Modifications:
-Correct the ordering of initialization and building to avoid NPE.

Result:
No more NPE in  construction.
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.
Changing HttpHeaders.Names to lower case