A way to force re-sync a folder

Please note that this approach doesn't currently work well with outlook.
If we apply this approach to e.g. the Calendar folder we end up with a
temporarily broken state (it seems that outlook can't deal with the same
collection getting removed and readded in quick succession, but that's a
guess).
In any case, it seems that restarting outlook eventually fixes things.

I'm adding this to the codebase even if it's maybe not a great solution
in production due to the above, but it's very useful to reproduce the
described synchronization problem.

Reviewers: Syncroton Developers

Subscribers: Syncroton Developers

Differential Revision: https://git.kolab.org/D4998