soledad sync issues with big amounts of email

Stressing bitmask a bit (30 emails with attachments) the backend freezes and stop doing anything. I don't see anything in the logs.

(from redmine: created on 2015-07-06, closed on 2015-11-03, relates #6884 (closed), relates #7216 (closed), relates #7178 (closed), relates #7318 (closed))