imap_tester: Fixed build after Protocol.GetFolders() changes.
imap: Testing for new messages is now working.

* There is now a CheckSubscribedFoldersCommand that is issued on the main
  connection that triggers everything.
* The new CheckMailboxesCommand divides the new mail check into several
  portions, and requeues itself until the next step, ie. the
  FetchHeadersCommand.
* The headers of the new mails are downloaded, but nothing is done with
  them yet.
* The actual check for the new mails doesn't scale that well yet, not sure
  how to properly do this without having to rely on the mail indices. Might
  be sensible to solve this via some simple heuristic.