IMAP Gateway: Implement IMAP PUSH (IDLE Mode)

[Andreas Hügel]

implement the IDLE Mode Extension

[Andreas Hügel (andreas-huegel)]

Todos
- change parser
- allow responses to be sent even if a tagged response does not follow. Maybe implement a response flush mechanism, that can be triggered on purpose. Keep in mind, that some untagged responses may not be sent, if the command in whole will fail. E.g. Exists in the select command
- work out this mismatch:
-- this is an extract of the RFC2177: ... As in the base specification, the processing of any new
   command may cause the sending of unsolicited untagged responses,
   subject to the ambiguity limitations. ...
-- later in the RFC2177 an example shows, that no untagged responses are sent in an Fetch command, even though a change has taken place after idle mode had been stopped.
-- maybe just try, what thunderbird will do.
-- problem is, if we cant send them in the fetch command, all of the sudden we have to buffer such information somewhere or stop the system from processing them in these cases.