pgrowlocks: Use GetActiveSnapshot() rather than SnapshotNow.

Per discussion, it's desirable to eliminate all remaining uses of
SnapshotNow, because it has unpleasant semantics: race conditions
can result in seeing multiple versions of a concurrently updated
row, or none at all.  By using GetActiveSnapshot() here, callers
will see exactly those rows that would have been visible if the
invoking query had scanned the table using, for example, a SELECT
statement.

This is slightly different from the old behavior, because commits
that happen concurrently with the scan will not affect the results.
In REPEATABLE READ or SERIALIZABLE modes, where transaction
snapshots are used, commits that have happened since the start of
the transaction will also not affect the results.  It is hoped
that this minor incompatibility will be thought an improvement,
or at least no worse than what we did before.