By default, we check the length of response body matches
content-length.  For HEAD request, this is not necessarily true, so we
sniff request method, and if it is HEAD, make sure that response body
length is 0.  But this does not work for HTTP Upgrade, since
nghttp2_session_upgrade() has no parameter to tell the request method
was HEAD.  This commit disables this response body length validation
for the stream upgraded by HTTP Upgrade.  We will add new version of
nghttp2_session_upgrade with the parameter to pass the request method
information so that we can handle this situation properly.

For both HTTP/2 frontend and backend.

Also adds http2::stringify_status to optimize status code
serialization.

For first scheduling, we might ignore weight.  This is OK since weight
is just a distribution of resource, rather than strict priority
ordering.

With 4825009e, static lib is no longer
required for unit tests.

This function is useful for the client application to know that there
is a chance that request can be sent.  If this function returns 0,
there is zero chance to make a request.

This commit also set error_code passed to
nghttp2_on_stream_close_callback to NGHTTP2_REFUSED_STREAM if request
is not sent.

If request submission is failed, make all remaining requests for that
client fail.

User can change file name using --mime-types-file option.

It seems that using ev_feed_event to signal write operation is much
faster than starting watcher.  This is probably due to the fact that
we don't need to wait in event loop.  The same thing cannot be done in
HTTP/2 frontend, since this will raise write operation for each stream
HEADER/DATA write, which leads to very small packets, hurting
performance.  Interestingly, HTTP/1 frontend also suffers the same
performance hit.

Revert "add PIDFile directive to systemd service"

h2load: do not overwrite protocol value

This reverts commit 19c095a8.

See discussion at #375 for details.

This commit also fixes the problem that application protocol is not
shown if cleartext spdy is used.