caProvider: speed up monitor list handling

Not a major speed-up since the list of monitors is per-channel,
but using wptr::expired() is more efficient than wptr::lock().

src/ca/caChannel.cpp

@@ -300,10 +300,10 @@ void CAChannel::addMonitor(CAChannelMonitorPtr const & monitor)
 {
     std::vector<CAChannelMonitorWPtr>::iterator it;
     for (it = monitorlist.begin(); it!=monitorlist.end(); ++it) {
-        CAChannelMonitorWPtr mon = *it;
-        if (mon.lock()) continue;
-        mon = monitor;
-        return;
+        if (it->expired()) {
+            *it = monitor;
+            return;
+        }
     }
     monitorlist.push_back(monitor);
 }