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.

This change will utilize last_stream_id in GOAWAY extensively.  When
GOAWAY is received with a last_stream_id, library closes all outgoing
streams whose stream_id > received last_stream_id.
nghttp2_on_stream_callback is called for each stream to be closed.

When GOAWAY is sent with a last_stream_id, library closes all incoming
streams whose stream_id > sent last_stream_id.
nghttp2_on_stream_callback is called for each stream to be closed.