pgsql-server: Renumber SnapshotNow and the other special snapshot codes

Log Message:
-----------
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.

Modified Files:
--------------
pgsql-server/src/backend/access/heap:
heapam.c (r1.173 -> r1.174)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/heap/heapam.c.diff?r1=1.173&r2=1.174)
pgsql-server/src/backend/commands:
async.c (r1.116 -> r1.117)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/async.c.diff?r1=1.116&r2=1.117)
pgsql-server/src/backend/executor:
execMain.c (r1.236 -> r1.237)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/execMain.c.diff?r1=1.236&r2=1.237)
execUtils.c (r1.114 -> r1.115)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/execUtils.c.diff?r1=1.114&r2=1.115)
pgsql-server/src/include/utils:
tqual.h (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/utils/tqual.h.diff?r1=1.50&r2=1.51)