Initially, we use nghttp2_stream.data_item to refer only item with
DATA frame.  But recently we use it to refer HEADERS frame as well.
So it is better to call just item rather than data_item.  This applies
to all related functions.

Previously session_after_frame_sent is called after we detected all
data is sent.  In nghttp2_session_mem_send, we only detect it in the
next call of the function.  It means that if a frame data bearing
END_STREAM is on flight to the peer as a result of
nghttp2_session_mem_send, peer may get that data and knows the stream
closure and issues new stream.  We may receive this new stream before
the next nghttp2_session_mem_send call, which means that we may
incorrectly assumes that peer violates maximum concurrent stream
limit.  To fix this issue, we separate session_after_frame_sent into 2
functions: session_after_frame_sent1 and session_after_frame_sent2.
session_after_frame_sent1 handles on_frame_send_callback and stream
closure and we call this early in nghttp2_session_mem_send.  This
makes number of streams are synchronized correctly with peer.

The nghttp2 library itself is still h2-14.  To experiment with the
implementations to require h2-16 to test new features (e.g.,
prioritization), nghttp, nghttpx, nghttpd and h2load now support h2-16
as well as h2-14.  Cleartext HTTP Upgrade is still limited to h2-14
however.