Renumber SnapshotNow and the other special snapshot codes so that

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 493f72606b463a75ae4e2ee4e64d829e7f56066d
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2004-09-11T18:28:34Z
Releases: 8.0.0
Renumber SnapshotNow and the other special snapshot codes so that
((Snapshot) NULL) can no longer be confused with a valid snapshot,
as per my recent suggestion.  Define a macro InvalidSnapshot for 0.
Use InvalidSnapshot instead of SnapshotAny as the do-nothing special
case for heap_update and heap_delete crosschecks; this seems a little
cleaner even though the behavior is really the same.

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