http2: discard frames with no SessionHandle

Return 0 instead of NGHTTP2_ERR_CALLBACK_FAILURE if we can't locate the
SessionHandle. Apparently mod_h2 will sometimes send a frame for a
stream_id we're finished with.

Use nghttp2_session_get_stream_user_data and
nghttp2_session_set_stream_user_data to identify SessionHandles instead
of a hash.

Closes #372