Fix crash
nghttpx: Add stream level timeout for HTTP/2 and SPDY upstream/downstream
nghttpx: Lower timeouts
nghttpx: Make stream timeout disable by default

It might be useful to clean the unused stream out to make up the room
for new streams.  On the other hand, proxy should maintain the
connection between upstream client and downstream server and they have
the timeout for their own.  Proxy just reacts to their decision.
Add note how to specify error code on NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE