* Switched all operations on the list of clients that could be either sync or async to use Scene.ForEachClient() instead of referencing ClientManager directly

* Added a new [Startup] config option called use_async_when_possible to signal how to run operations that could be either sync or async
* Changed Scene.ForEachClient to respect use_async_when_possible
* Fixing a potential deadlock in Parallel.ForEach by locking on a temporary object instead of the enumerator (which may be shared across multiple invocations on ForEach). Thank you diva

OpenSim/Framework/Parallel.cs

@@ -118,6 +118,7 @@ namespace OpenSim.Framework
             int counter = threadCount;
             AutoResetEvent threadFinishEvent = new AutoResetEvent(false);
             IEnumerator<T> enumerator = enumerable.GetEnumerator();
+            object syncRoot = new object();
             Exception exception = null;
 
             for (int i = 0; i < threadCount; i++)
@@ -131,7 +132,7 @@ namespace OpenSim.Framework
                         {
                             T entry;
 
-                            lock (enumerator)
+                            lock (syncRoot)
                             {
                                 if (!enumerator.MoveNext())
                                     break;