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.

Previously we handle idle streams as closed streams.  We only keeps
sum of closed streams and active streams under max concurrent streams
limit, idle streams gets deleted earlier than client expects.

In this change, idle streams are kept in separate list and not handled
as closed streams.  To mitigate possible attack vector to make
unlimited idle streams, we cap the number of idle streams in a half of
max concurrent streams.  This is arbitrary choice.  It may be adjusted
in the future when we have interop experience.

Previously when requests are issued to HTTP/2 downstream connection,
but it turns out that connection is down, handlers of those requests
are deleted.  In some situations, we only know connection is down when
we write something to network, so we'd like to handle this kind of
situation in more robust manner.  In this change, certain seconds
passed after last network activity, we first issue PING frame to
downstream connection before issuing new HTTP request.  If writing
PING frame is failed, it means connection was lost.  In this case,
instead of deleting handler, pending requests are migrated to new
HTTP2/ downstream connection, so that it can continue without
affecting upstream connection.

nghttp2_mem structure is introduced to hold custom memory allocator
functions and user supplied pointer.  nghttp2_mem object can be passed
to nghttp2_session_client_new3(), nghttp2_session_server_new3(),
nghttp2_hd_deflate_new2() and nghttp2_hd_inflate_new2() to replace
standard malloc(), free(), calloc() and realloc().  nghttp2_mem
structure has user supplied pointer mem_user_data which can be used as
per session/object memory pool.