src: Set errno = 0 before calling strtol
nghttpx: Propagate upstream priority change to downstream
Don't return NGHTTP2_ERR_STREAM_CLOSED when submitting DATA, PRIORITY, WU

Remove the check to see that stream exists at the time when submitting
DATA, PRIORITY and WINDOW_UPDATE. We will do this check when we actually
serialize and send them off to the network (or application provided
buffer).
Make nghttp2_is_fatal public API
Code cleanup
nghttpx: Code cleanup
nghttpx: Use nghttp2_session_mem_recv instead of nghttp2_session_recv