Set cutoff xmin more aggressively when vacuuming a temporary table.

Since other sessions aren't allowed to look into a temporary table
of our own session, we do not need to worry about the global xmin
horizon when setting the vacuum XID cutoff.  Indeed, if we're not
inside a transaction block, we may set oldestXmin to be the next
XID, because there cannot be any in-doubt tuples in a temp table,
nor any tuples that are dead but still visible to some snapshot of
our transaction.  (VACUUM, of course, is never inside a transaction
block; but we need to test that because CLUSTER shares the same code.)

This approach allows us to always clean out a temp table completely
during VACUUM, independently of concurrent activity.  Aside from
being useful in its own right, that simplifies building reproducible
test cases.

Discussion: https://postgr.es/m/3490536.1598629609@sss.pgh.pa.us

src/include/commands/cluster.h

@@ -19,7 +19,8 @@
 
 
 extern void cluster(ClusterStmt *stmt, bool isTopLevel);
-extern void cluster_rel(Oid tableOid, Oid indexOid, int options);
+extern void cluster_rel(Oid tableOid, Oid indexOid, int options,
+						bool isTopLevel);
 extern void check_index_is_clusterable(Relation OldHeap, Oid indexOid,
 									   bool recheck, LOCKMODE lockmode);
 extern void mark_index_clustered(Relation rel, Oid indexOid, bool is_internal);

src/include/commands/vacuum.h

@@ -267,6 +267,7 @@ extern void vacuum_set_xid_limits(Relation rel,
 								  int freeze_min_age, int freeze_table_age,
 								  int multixact_freeze_min_age,
 								  int multixact_freeze_table_age,
+								  bool isTopLevel,
 								  TransactionId *oldestXmin,
 								  TransactionId *freezeLimit,
 								  TransactionId *xidFullScanLimit,