Finalsinks shouldn't wait for snapshots to complete.
Currently, if there's a snapshot in progress, and we want to launch a new one, finalsink will block until its done.
e.g. if calc_fap hasn't completed by the time the interval passes, finalsink will block until it can launch the next instance.
As far as I understand, it's not critical that snapshots are run on the dot. We can afford to wait a couple of minutes for a calc_fap, combine_stats, etc. to finish.
If a process hasn't completed by the time an interval passes, finalsink should just check again on the next buffer.