This commit also fixes defaltehd always reports output length is 0.

This identifier string is used if HTTP/2 is used over cleartext TCP.

NGHTTP2_CLIENT_CONNECTION_PREFACE has the same content with
NGHTTP2_CLIENT_CONNECTION_HEADER, which is now obsoleted by
NGHTTP2_CLIENT_CONNECTION_PREFACE.

pending_local_max_concurrent_stream is now set in
nghttp2_session_add_settings, rather than after frame was sent.

The number of closed stream to keep is limited by
MAX_CONCURRENT_STREAMS - current active stream.

This is useful because application may allocate resources for it and
wants to free the resources if they are not used anymore.

This change also reset read timeout when we have sent HEADERS,
PUSH_PROMISE or DATA.

Supplying multiple URIs can simulate more real life situation on
server side.  For example, we can supply URIs of html, css and js and
benchmark the server.  The -m option is updated so that it defaults to
the number of supplied URIs.