* Decouple AuthenticationEngine and BackendConnection

* Decouple AuthenticationEngine and BackendConnection

* revise java doc

* Add generic type for JDBCDatabaseCommunicationEngine

* Add generic type for ExecutionContext

* Add generic type for JDBCExecuteEngine

* Refactor JDBCExecutorWrapper

* Refactor JDBCExecutorWrapper

* Refactor StatementExecutorWrapper

* For code format