fix data reader uncolsed bug.

src/DotNetCore.CAP.MySql/MySqlStorageConnection.cs

@@ -150,7 +150,9 @@ SELECT * FROM `{_prefix}.received` WHERE Id=LAST_INSERT_ID();";
             try
             {
                 //fetchedMessage = await connection.QuerySingleOrDefaultAsync<FetchedMessage>(sql, args, transaction);
-                var reader = connection.ExecuteReader(sql, args, transaction);
+                // An anomaly with unknown causes, sometimes QuerySingleOrDefaultAsync can't return expected result.
+                using (var reader = connection.ExecuteReader(sql, args, transaction))
+                {
                     while (reader.Read())
                     {
                         fetchedMessage = new FetchedMessage
@@ -160,6 +162,7 @@ SELECT * FROM `{_prefix}.received` WHERE Id=LAST_INSERT_ID();";
                         };
                     }
                 }
+            }
             catch (MySqlException)
             {
                 transaction.Dispose();