Fixing bugs in HTTP/2 pipeline exception handling

Motivation:

HTTP/2 codec does not properly test exception passed to
exceptionCaught() for instanceof Http2Exception (since the exception
will always be wrapped in a PipelineException), so it will never
properly handle Http2Exceptions in the pipeline.

Also if any streams are present, the connection close logic will execute
twice when a pipeline exception. This is because the exception logic
calls ctx.close() which then triggers the handleInActive() logic to
execute.  This clears all of the remaining streams and then attempts to
run the closeListener logic (which has already been run).

Modifications:

Changed exceptionCaught logic to properly extract Http2Exception from
the PipelineException.  Also added logic to the closeListener so that is
only run once.

Changed Http2CodecUtil.toHttp2Exception() to avoid NPE when creating
an exception with cause.getMessage().

Refactored Http2ConnectionHandler to more cleanly separate inbound and
outbound flows (Http2ConnectionDecoder/Http2ConnectionEncoder).

Added a test for verifying that a pipeline exception closes the
connection.

Result:

Exception handling logic is tidied up.