Change currtid functions to use an MVCC snapshot, not SnapshotNow.

This has a slight performance cost, but the only known consumers
of these functions, known at the SQL level as currtid and currtid2,
is pgsql-odbc; whose usage, we hope, is not sufficiently intensive
to make this a problem.

Per discussion.
pgstattuple: Doc update for previous commit.

In my previous change to make pgstattuple use SnapshotDirty rather
than SnapshotNow, I failed to notice that the documenation also
needed to be updated to match.  Fix.