Locking on m_models in CreateModel and UnregisterModel.

RecoverModels method already locks on m_models collection. If CreateModel and UnregisterModel lock on a different object, then m_models collection can be modified while it is being read.
This is possible in concurrent scenarios, when the connection has been re-established, model recovery started, and then a new model created or disposed. In that case, there will be a "Collection was modified; enumeration operation may not execute" exception in RecoverModels and recovery will fail.

Author: Bojan Resnik

projects/client/RabbitMQ.Client/src/client/impl/AutorecoveringConnection.cs

@@ -539,7 +539,7 @@ public override string ToString()
 
         public void UnregisterModel(AutorecoveringModel model)
         {
-            lock (this)
+            lock (m_models)
             {
                 m_models.Remove(model);
             }
@@ -640,7 +640,7 @@ public IModel CreateModel()
             AutorecoveringModel m;
             m = new AutorecoveringModel(this,
                 CreateNonRecoveringModel());
-            lock (this)
+            lock (m_models)
             {
                 m_models.Add(m);
             }