We make following HEADERS under priority control:
* push response HEADERS
* HEADERS submitted by nghttp2_submit_response

Currently, HEADERS submitted by nghttp2_submit_headers is not attached
to stream.  This is because it may be used as non-final response
header and application may submit final response using
nghttp2_submit_response without checking non-final response header
transmission.

Allowing PRIORITY frame at anytime so that PRIORITY frame to idle
stream can create anchor node in dependency tree.  In this change, we
open stream with new NGHTTP2_STREAM_IDLE state, which is linked in
session->closed_stream_head and is treated as if it is closed stream.
One difference is that if the stream is opened, we remove it from
linked list and change the state to the appropriate one.  To O(1)
removal from linked list, we change session->closed_stream_head to
doubly linked list.

$alpn is a variable which represents ALPN identifier of the protocol
which generates the response.

$pid refers to the PID of the running process.

This is unfortunate but help2man behaves badly when there is indented
lines in help messages in commnad-line args.  We removed indentations
to make help2man happy.

time_iso8601 and request_time.

Add Upstream::on_handler_delete() hook to safely write log for
HttpsUpstream.

This commit adds functionality to customize access logging format in
nghttpx.  The format variables are inspired by nginx.  The default
format is combined format.