* Decouple AuthenticationEngine and BackendConnection

* Decouple AuthenticationEngine and BackendConnection

* Unify userName to username

* Refactor CommandExecutorTask

* Refactor MySQLComQueryPacketExecutor.isQueryResponse

* Refactor MySQLComStmtExecuteExecutor.isQueryResponse

* Refactor PostgreSQLComBindExecutor.isQueryResponse

* Refactor PostgreSQLComQueryExecutor.isQueryResponse