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 - The moving this to the testing suggestion was in response to your comment about it being useful for testing code.  I didn't think of the use case you described (sending after receiving something) and I agree this use-case seems main stream. 

An alternative solution may be creating a class that extends `Http2OutboundConnectionAdapter` and implements `Http2FrameListener`.  I think this will avoid the chicken/egg problem and also avoid the need for another layer in the listener chain.  I think the extension points we have now are really focused on the `Http2FrameListener` and the `Http2OutboundConnectionAdapter` while the other classes provide the infrastructure to tie these classes together.  First do you agree and second if you do...should we make this more apparent/restrictive (i.e. make `Http2ConnectionHandler ` a `final Http2ConnectionHandler ` class)

No I don't have an example that needs to do this.