Drop rconn's responsibility for limiting the tx queue.

Now it helps clients track the number of in-flight messages, but lets the
clients do the limiting themselves.  This will come in handy for packet-in
rate limiting (in an upcoming commit), in which we want to track in-flight
packet-in messages separately from other in-flight messages.

controller/controller.c

@@ -197,7 +197,7 @@ main(int argc, char *argv[])
 static void
 new_switch(struct switch_ *sw, struct vconn *vconn, const char *name)
 {
-    sw->rconn = rconn_new_from_vconn(name, 128, vconn);
+    sw->rconn = rconn_new_from_vconn(name, vconn);
     sw->lswitch = lswitch_create(sw->rconn, learn_macs,
                                  setup_flows ? max_idle : -1);
 }