CFT: fix OfflineIMAP lockup in single-threaded mode

[Eygene Ryabinkin]

G'day.

I was trying to integrate the fix for the issue #22,
  https://github.com/OfflineIMAP/offlineimap/issues/22
but came up with the updated patch,
  http://codelabs.ru/patches/offlineimap/2012-preliminary-fix-deadlock-singlethreaded-IMAP-sync.diff

In a nutshell, if you're syncing more than 100 (accounts + messages),
you will experience a lockup if offlineimap is executed with flag
"-1".  This patch tries to catch all such problems.  It is not yet in
the 'next' branch, since I'll be polishing it a bit, but the wider
testing for it is welcome, both in single-threaded (via "-1") and your
regular mode of operations.  Comments and reviews are welcome too.

Thanks!
-- 
rea