Previously we do not specify the number of requests each client has to
issue.  The each client corresponds to 1 TCP connection.  If
connection was not accepted by server or not TLS handshake is not
done, we effectively don't use that connection and the requests
supposed to be issued for those connections are done via other
established connections.  If this occurs, servers which do not accept
all connections may gain good benchmark results since they don't have
to pay extra cost to handle all connections (e.g., SSL/TLS handshake).
This change explicitly set the number of requests each client has to
issue so that servers cannot *cheat*.

Previously in inflater we reserve new ringbuffer when table size is
changed.  This may be potentially a problem if new table size is very
large number.  When inflater is not used directly by application, this
is not a problem because application can choose the buffer size.  On
the other hand, if application uses inflater directly and it does not
have control of new buffer size (e.g., protocol dissector), then we
just fail to allocate large buffer in
nghttp2_hd_inflate_change_table_size() without actually use such huge
buffer.  This change defers the actual allocation of buffer when it is
actually needed so that we will fail when it is absolutely needed.

It looks like setting read-rate and read-burst to 0 makes busy loop.
It seems a bug.  On the other hand, we most likely want per-thread
rate limit rather than per-connection.  So we decided to drop them.

It seems that if readcb is not set before SSL/TLS handshake, the
incoming data already available when eventcb (BEV_EVENT_CONNECTED
event) is fired is not further notified after setting new readcb.  We
knew this fact and call upstream->on_read() in eventcb, but it is
wrong for HTTP/2.  We have to call upstream_http2_connhd_readcb to
check connection preface.  Otherwise, we consume it by nghttp2 session
and it is treated as unknown frame and connection preface is not
detected properly.

Add last_stream_id parameter to nghttp2_submit_goaway().  To terminate
connection immediately with application chosen last stream ID,
nghttp2_session_terminate_session2() was added.

This commits also fixes last stream ID in GOAWAY uses wrong stream ID.

Thanks for Pascal

Make the check for a valid index range a macro, so the compiler doesn't
whine if it's not used, but it's available if it *is* used.