Merge branch 'include_pull_request' of https://github.com/alex-nalivko/nghttp2 into alex-nalivko-include_pull_request

Merge branch 'finer_stats' of https://github.com/ericcarlschwartz/nghttp2 into ericcarlschwartz-finer_stats

finer statistics for h2load: update per comments from tatsuhiro-t

finer stats for h2load: fixed formatting

We have to return this error code to notify OpenSSL that we have not
filled any values in |*out| and |*outlen|.

Previously h2load used default flow control window as described in
HTTP/2 and SPDY specification.  The window size is 64KiB, which is a
bit small, and cannot utilize full server performance when response
size is not too small.  Basically, we do this kind of benchmarking
test to measure server's throughput, and optimal performance.  Smaller
window certainly degrades performance even in local testing because
server is so fast that it has to wait for WINDOW_UPDATE from h2load.
To make default behaviour suitable for peak performance test, we
decided to disable flow control in h2load by setting large enough
window size.

Most users used h2load without -w or -W options, so they were
implicitly throttled by flow control and the result was affected by
that negatively.  Now flow control is disabled by default, the result
may improve depending on the implementations.

After reviewing codebase, only queue for DATA frames requires
priorities.  Other frames can be replaced multiple linear queues.
Replacing priority queue with linear queue allows us to simplify
codebase a bit; for example, now nghttp2_session.next_seq is gone.