Fetcher opens directory in readonly mode, so we don't mark messages as read

Also, source param added for copy/move
Mover action fix, fetcher num_emails can performance improved

* Mover action: don't expunge immediately after move, since it breaks
  email ids for fetcher.
* num_emails: no separate search necessary to find out all message ids,
  just use the return of .select() that gives number of messages in directory