Grab-bag of minor changes to HTTP/2

Motivation:

Various small fixes/improvements to the interface to the HTTP/2 classes,
as well as some minor performance improvements.

Modifications:

- Added fix for IntObjectHashMap to ensure that capacity is always odd.
Even capacity can cause probing to fail.
- Cleaned the access to GOAWAY information in Http2Connection interface.
Endpoints now manage their own state for GOAWAY. Also added a goingAway
event handler.
- Added Endpoint methods for checking MAX_CONCURRENT_STREAMS or if the
number of streams for the endpoint have been exhausted. See
Endpoint.nextStreamId()/acceptingNewStreams().
- Changed DefaultHttp2Connection to use IntObjectHashMap. This should be
a slight memory improvement.
- Fixed check for MAX_CONCURRENT_STREAMS to correctly use the number of
active streams for the endpoint (not total active). See
DefaultHttp2Connection.checkNewStreamAllowed.
- Exposing a few methods to subclasses of AbstractHttp2ConnectionHandler
(e.g. exception handling).
- Cleaning up GOAWAY and RST_STREAM handling in
AbstractHttp2ConnectionHandler.

Result:

HTTP/2 code should provide more information to subclasses and will have
a reduced memory footprint.