MEDIUM: h2: Don't use a wake() method anymore.

Instead of having our wake() method called each time a fd event happens,
just subscribe to recv/send events, and get our tasklet called when that
happens. If any recv/send was possible, the equivalent of what h2_wake_cb()
will be done.
diff --git a/src/connection.c b/src/connection.c
index ad03863..b970d41 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -229,7 +229,7 @@
 	if ((io_available || (((conn->flags ^ flags) & CO_FL_NOTIFY_DATA) ||
 	     ((flags & (CO_FL_CONNECTED|CO_FL_HANDSHAKE)) != CO_FL_CONNECTED &&
 	      (conn->flags & (CO_FL_CONNECTED|CO_FL_HANDSHAKE)) == CO_FL_CONNECTED))) &&
-	    conn->mux->wake(conn) < 0)
+	    conn->mux->wake && conn->mux->wake(conn) < 0)
 		return;
 
 	/* commit polling changes */