Avoid missing sync intervals

Summary:
We tried to guarantee that we never sync more frequently than the
minimum sync interval. It can happen though that the timer fires a
little bit early, resulting in a complete sync interval being missed.

We avoid this problem by only not syncing again if we did sync within
(interval - 2min).

Phabricator: git.kolab.org/T1295

Reviewers: knauss

Differential Revision: https://git.kolab.org/D189