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.

For HTTP/1 backend, -b option can be used several times to specify
multiple backend address.  HTTP/2 backend does not support multiple
addresses and only uses first address even if multiple addresses are
specified.

This commit limits the number of concurrent HTTP/1 downstream
connections to same host.  By defualt, it is limited to 8 connections.
--backend-connections-per-frontend option was replaced with
--backend-http1-connections-per-host, which changes the maximum number
of connections per host.  This limitation only kicks in when h2 proxy
is used (-s option).

This makes the library h2-16 compatible now.