Use InvalidSnapshot, now SnapshotNow, as the default snapshot.

As far as I can determine, there's no code in the core distribution
that fails to explicitly set the snapshot of a scan or executor
state.  If there is any such code, this will probably cause it to
seg fault; friendlier suggestions were discussed on pgsql-hackers,
but there was no consensus that anything more than this was
needed.

This is another step towards the hoped-for complete removal of
SnapshotNow.