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.

Added a test for verifying that a pipeline exception closes the
connection.

Result:

Exception handling logic is tidied up.
@nmittler - I'm still not sure this class solves the chicken and the egg problem.  The self escaping issue in the constructor still exists.  What if the `super` constructor uses the listener object which has not been fully initialized yet (I know it doesn't right now, and this may not be a likely future addition right now)?  The existing `Http2ConnectionHandler` has some simple constructors which take a `Http2FrameListener ` as an argument so I don't think you need to extend the pipeline for the simple cases.  If it does exist to make testing easier can we move it into the testing code?