Automatically run task gc after sync

[djmitche]

Sync can potentially change the set of pending tasks, so it should update the working set. It's also the point where we know the replica's data is backed up, so it seems like a good time for garbage collection.

[djmitche]

I think we'll want to have a number of maintenance operations:

• adding pending tasks to the end of the working set, without renumbering
• renumbering (compacting) the working set
• sync'ing
• expiring deleted / completed tasks N days after their last modification

Probably only the first of those should happen after a sync. TaskWarrior renumbers on every list operation, so we cold replicate that behavior.