Sends a 'CHECK' command to the server.
The 'callback' parameter specifies a function which will be called when the operation completes (or an error is encountered). If the 'callback' parameter is not specified, then the method will block and will not return until the operation completes (or an error is encountered).
The callback for this method is defined as:
'err' is the error that occurred. If there was no error, then 'err' is 'null'.
'err' has 2 properties which hold detailed information:
This method sends a 'CHECK' command to the server, asking it to perform any necessary 'housekeeping' operations on, and update the state of the currently selected mailbox.