nghttpx: Support per-backend mruby script

Fix stream reset if data from client is arrived before dconn is attached

Tweak nghttp2_session_set_stream_user_data

nghttp2_session_set_stream_user_data now works for a stream which is
not created yet, but the request which creates the stream is queued.

Fix handling of SETTINGS_MAX_CONCURRENT_STREAMS.

Remove remaining SPDY code from the integration tests.

The maximum number of outgoing concurrent streams is initially
limited to 100 to avoid issues when the local endpoint submits
lots of requests before receiving initial SETTINGS frame from
the remote endpoint, since sending them at once to the remote
endpoint could lead to rejection of some of the requests.

This initial limit is overwritten with the value advertised in
SETTINGS_MAX_CONCURRENT_STREAMS setting by the remote endpoint,
but previously, it wasn't lifted if the remote endpoint didn't
advertise that setting (implying no limits), in which case the
limit of 100 was retained, even though it was never advertised
by the remote endpoint.
Signed-off-by: Piotr Sikora <piotrsikora@google.com>

Signed-off-by: Piotr Sikora <piotrsikora@google.com>