server: check tx buffer limit to throttle

The TX buffer could grow while nothing is being received.
Practically bounded by the timeout interval, but could
still get quite large in that time.

src/servermon.cpp

@@ -90,7 +90,9 @@ struct MonitorOp final : public ServerOp
                 if(!conn || conn->state==ConnBase::Disconnected)
                     return;
 
-                if(conn->connection() && (bufferevent_get_enabled(conn->connection())&EV_READ)) {
+                auto bev(conn->connection());
+
+                if(bev && evbuffer_get_length(bufferevent_get_output(bev)) < conn->tcp_tx_limit) {
                     doReply(op);
                 } else {
                     // connection TX queue is too full