Add support for task expiration

[djmitche]

This adds optional support for task expiration, implemented by calling the relevant Replica method.

This is currently fixed at 180 days, but we can add configuration later.

I chose to do this on sync for two reasons:

• Any modifications to tasks from other replicas are likely to have just been received, updating the modified timestamp and saving a task from being updated-and-deleted. Such a task will end up in a zombie state with only the updated properties surviving.
• This is a fairly expensive operation (scan of the full DB) so not something we should do interactively, like renumbering the working set.

Fixes #3402.

[felixschurk]

I looked over it and as far as I can understand everything works as expected and is documented.

[djmitche]

I looked over it and as far as I can understand everything works as expected and is documented.

I can't ask for anything more than that in a review!

[felixschurk]

Interesting that here the tests all run fine, however when merged into develop (213b9d3) the tests fail in some of the runners.

I don't understand why and how.

[djmitche]

Huh, I re-ran them and they passed. If they continue to fail we can try reverting this, but it really seems unrelated.

[felixschurk]

Yes, I don't think it has to do with the changes.
But still strange.