LibIPC: Only start responsiveness timer after sending client a message

Instead of always running the responsiveness timer for IPC clients,
we now only start it after sending a message. This avoids waking up
otherwise idle clients to do ping/pong busywork.

Services/WindowServer/ClientConnection.cpp

@@ -844,4 +844,9 @@ void ClientConnection::may_have_become_unresponsive()
     });
 }
 
+void ClientConnection::did_become_responsive()
+{
+    set_unresponsive(false);
+}
+
 }