Drops unnecessary logs for subscribers, to reduce memory usage. In local
testing, this incurred a 20% improvement in graph-store memory usage.
Note that publishers still retain the entire history, for backlog and
resubscribe purposes. Subscribers still retain a handful of updates so
that they can resubscribe upon kick correctly.
If a subscription was kicked before receiving any graph-updates on it,
then the attempt to construct a valid resubscribe path would fail, as
the scry crashes. Fixes this by handling the missing update-log case
correctly.
