Trade recursion by plain old cycle

We can do a simpler and more stack-friendly hack for IMAP
servers with limited line lenghts.

Reported-by: Josh Berry, https://github.com/josh-berry
GH: https://github.com/OfflineIMAP/offlineimap/pull/100
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>