Switch cowboy_http2 to the new cow_http2_machine

The new module is a merge of the Cowboy and Gun HTTP/2
state machines. Using a common code will help future
developments rather than duplicating the work.

A notable change is in how streams are terminated
when the handler stops before the body is sent. The
cowboy_stream:terminate function is now called only
after the body has been sent fully (or the stream
is reset in-between), not when the stop command is
returned. This will most likely have an impact on
metrics but will be closer to reality.

I had to comment a broken test in rfc7231_SUITE that
was cheating, cheating is no longer possible.

This depends on Cowlib master for the time being. A
new Cowlib version will be released once both Cowboy
and Gun are ported to use cow_http2_machine and I'm
satisfied with it.

src/cowboy_metrics_h.erl

@@ -71,8 +71,7 @@
 	%% Start/end of the processing of the request.
 	%%
 	%% This represents the time from this stream handler's init
-	%% to terminate. Note that this doesn't indicate the response
-	%% has been sent fully, it still may be queued up in a buffer.
+	%% to terminate.
 	req_start => integer(),
 	req_end => integer(),