http2: Set SSN active on create_stream

[masaori335]

Prior to the change, if a HTTP/2 stream is open on a session, but it doesn't make new transaction by some reason, the inactivity timeout of the connection never get fired.

Current ATS v10.0.x cancels inactivity timeout of proxy session in the Http2ConnectionState::create_stream (introduced by 7d5e673). We need to get back the session inactivity timeout when all streams are gone, but there is a activity check of proxy session. To pass the check, we need to make the proxy session active.

trafficserver/src/proxy/http2/Http2ConnectionState.cc

Lines 1994 to 1998 in 66940f5

	} else if (session->get_proxy_session()->is_active()) {
	// If the number of clients is 0, HTTP2_SESSION_EVENT_FINI is not received or sent, and session is active,
	// then mark the connection as inactive
	session->do_clear_session_active();
	session->set_no_activity_timeout();

[cmcfarlen]

Cherry-picked to v10.0.x