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.