Fix flush unclaer data bugs.

src/DotNetCore.CAP/ICapTransaction.Base.cs

@@ -24,12 +24,14 @@ namespace DotNetCore.CAP
             _bufferList.Add(msg);
         }
 
-        protected void Flush()
+        protected virtual void Flush()
         {
             foreach (var message in _bufferList)
             {
                 _dispatcher.EnqueueToPublish(message);
             }
+
+            _bufferList.Clear();
         }
 
         public abstract void Commit();