caProvider: Speed up addChannel()

Use wptr::expired() instead of wptr::lock() for testing if the
shared pointer is still valid, *much* faster!
Also switched from index to interator, but that's incidental.

src/ca/caProvider.cpp

@@ -107,12 +107,12 @@ Channel::shared_pointer CAChannelProvider::createChannel(
 
 void CAChannelProvider::addChannel(const CAChannelPtr &channel)
 {
+    std::vector<CAChannelWPtr>::iterator it;
     epicsGuard<epicsMutex> G(channelListMutex);
-    for (size_t i = 0; i < caChannelList.size(); ++i)
+    for (it = caChannelList.begin(); it != caChannelList.end(); ++it)
     {
-        if (!(caChannelList[i].lock()))
-        {
-            caChannelList[i] = channel;
+        if (it->expired()) {
+            *it = channel;
             return;
         }
     }