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.
@Scottmitch I don't think the duplication here is all that bad.  First off, it should be noted that I'm not entirely sure we need to override exceptionCaught() at all, since we don't have other upstream handlers that will be throwing Http2Exception. Originally we copied the SPDY implementation which had other upstream handlers that could throw and this was a way of communicating errors between handlers ... not sure it still applies.

Regardless ... onException() is called from 2 other places, neither of which will need to conditionally call onException().  This is a special case where we only want to call onException if we're sure the exception came from the HTTP/2 layer. I think it's reasonable to just have this done here even if it is a slight duplication of what is done inside onException().  WDYT?