Lock explicitly

Instead of relying on concurrent session usages to do it through BeginProcess.

Author: fredericDelaporte

src/NHibernate/Transaction/AdoNetWithSystemTransactionFactory.cs

@@ -484,6 +484,7 @@ protected virtual void CompleteTransaction(bool isCommitted)
 				// do an early exit here in such case.
 				if (!IsInActiveTransaction)
 					return;
+				Lock();
 				var isSessionProcessing = _session.IsProcessing();
 				try
 				{