Set cutoff xmin more aggressively when vacuuming a temporary table.

 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