SMTPConnection clean up

[gaol]

Code clean up

• The following handlers are used in SMTPConnection to handle success response and error, they should be changed to Handler<AsyncResult<String>> handler instead. The implementation in vertx-mqtt-client can be referenced.

  private Handler<String> commandReplyHandler;
  private Handler<Throwable> errorHandler;

• The Handler<String> initialReplyHandler should be updated to Handler<AsyncResult<String>> initialReplyHandler

  void init(Handler<String> initialReplyHandler)

• The private Handler<Throwable> errorHandler should be removed
• The void write(String str, Handler<String> commandResultHandler) should be changed to void write(String str, Handler<AsyncResult<String>> commandResultHandler)

Overall any operation that has an Handler<T> should be Handler<AsyncResult<T>>