pgsql: Use InvalidSnapshot, now SnapshotNow, as the default snapshot.

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/765ad89be36f699e2d65238c1f0458a1ab7e4d8b

Modified Files
--------------
src/backend/access/index/genam.c | 2 +-
src/backend/executor/execUtils.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)