We added spdylay build since integration tests require it.

This avoid establishing HTTP/2 backend connection again w/o pending
request.

See GH-431

The added API is nghttp2_session_change_stream_priority().  This
provides the same functionality to re-prioritize stream when PRIORITY
frame.  is received, but we do it without PRIORITY frame.  This could
be useful for server to change pushed stream's priority silently.

This commit adds ability to check status of cached file descriptor to
make sure that it can be reused.  We inspect last modification time
and number of hard links.  If last modification is changed from the
last validation time, or number of hard links gets 0, we don't reuse
file descriptor.  We also capped upper limit of the cached file
descriptors.  If the limit is reached, we will close file descriptor
which is least recently used, and its usecount is 0.

This is very simple API, and it returns nonzero if session is
initialized as server.

When stream is removed from tree, stream is either closed, or its
remote flow control window is depleted.  In the latter case, we
schedule this stream as fast as possible if its remote window gets
positive, since it did not sent anything in its turn.  To achieve
this, reset last_writelen to 0 when stream is removed from tree.

When connection is dropped, we get "nonzero" error code ec in
read_socket callback.  We can just call on_error callback if ec is
nonzero.  But there is a problem when we did ordered shutdown.  In
this case, we also get EOF in ec, but we don't want to call on_error
because it is not an error.  To workaround this, we check return value
of should_stop().  If it is true, then we assume that HTTP/2 session
cleanly ended, and we don't call on_error.

To make use cache fd more robust manner (e.g. among several
connections), eviction of cached file descriptor now takes place using
timer.  The timer is started when there is no handler (no
connections).  The timeout value is hard-coded and 2 seconds.

This commit allows user to override user-agent with -H option.  We
also enabled custom header support for http/1.1 requests.  We also did
minor optimizations (std::vector::reserve).

This commits enables HTTP/2 server push from HTTP/2 backend to be
relayed to HTTP/2 frontend.  To use this feature, --http2-bridge or
--client is required.  Server push via Link header field contiues to
work.

For clients, CANCEL is more appropriate for both incoming/outgoing
streams.  For servers, CANCEL is appropriate for its pushed stream
(outgoing), but REFUSED_STREAM is more appropriate for incoming
stream.

Apply typo fixes to the original source files in the previous commit
which were done in generated text.

This is a regression when we introduced SSL/TLS session resumption in
HTTP/2 backend.  Before the introduction of session resumption,
conn_.tls.ssl is always nullptr when connection is made to proxy.  But
we have to keep conn_.tls.ssl to enable session resumption, so our
code breaks when it is reused.  This commit fixes this issue.

See GH-421