Fixes#8212
Some snapshotting operations call in on a single table at a time.
When checking for existing snapshots in this case, we should not
bother with snapshots in other tables. Add an optional "filter"
to check routine, which if non-empty includes tables to check.
Use case is "scrub" which calls with a limited set of tables
to snapshot.
Closes#8240
(cherry picked from commit f44420f2c9)
b1032a2699