The parsing was broken when the first character of the header name was invalid.

Based on a patch by Alan Kemp.

Previously, "/foo///../bar" was normalized into "/foo/bar"
instead of "/foo//bar".

This could happen when graceful shutdown configured by worker_shutdown_timeout
times out and is then followed by another timeout such as proxy_read_timeout.
In this case, the HEADERS frame is added to the output queue, but attempt to
send it fails (due to c->error forcibly set during graceful shutdown timeout).
This triggers request finalization which attempts to close the stream.  But the
stream cannot be closed because there is a frame in the output queue, and the
connection cannot be finalized.  This leaves the connection open without any
timer events leading to alert.

The fix is to post write event when sending output queue fails on c->error.
That will finalize the connection.

With this patch, all traffic over an HTTP/2 connection is counted in
the h2c->total_bytes field, and payload traffic is counted in
the h2c->payload_bytes field.  As long as total traffic is many times
larger than payload traffic, we consider this to be a flood.

In 8df664ebe037, we've switched to maximizing stream window instead
of sending RST_STREAM.  Since then handling of RST_STREAM with NO_ERROR
was fixed at least in Chrome, hence we switch back to using RST_STREAM.

This allows more effective rejecting of large bodies, and also minimizes
non-payload traffic to be accounted in the next patch.

Don't waste server resources by sending RST_STREAM frames.  Instead,
reject WINDOW_UPDATE frames with invalid zero increment by closing
connection with PROTOCOL_ERROR.