This fixes a bug in the edge case where, for a temp table, heap_page_prune()
can end up with a different horizon than heap_vacuum_rel(). Which can trigger
errors like "ERROR: cannot freeze committed xmax ...".

The bug was introduced due to interaction of a7212be8 "Set cutoff xmin more
aggressively when vacuuming a temporary table." with dc7420c2 "snapshot
scalability: Don't compute global horizons while building snapshots.".

The problem is caused by lazy_scan_heap() assuming that the only reason its
HeapTupleSatisfiesVacuum() call would return HEAPTUPLE_DEAD is if the tuple is
a HOT tuple, or if the tuple's inserting transaction has aborted since the
heap_page_prune() call. But after a7212be8 that was also possible in other
cases for temp tables, because heap_page_prune() uses a different visibility
test after dc7420c2.

The fix is fairly simple: Move the special case logic for temp tables from
vacuum_set_xid_limits() to the infrastructure introduced in dc7420c2. That
ensures that the horizon used for pruning is at least as aggressive as the one
used by lazy_scan_heap(). The concrete horizon used for temp tables is
slightly different than the logic in dc7420c2, but should always be as
aggressive as before (see comments).

A significant benefit to centralizing the logic procarray.c is that now the
more aggressive horizons for temp tables does not just apply to VACUUM but
also to e.g. HOT pruning and the nbtree killtuples logic.

Because isTopLevel is not needed by vacuum_set_xid_limits() anymore, I
undid the the related changes from a7212be8.

This commit also adds an isolation test ensuring that the more aggressive
vacuuming and pruning of temp tables keeps working.
Debugged-By: Amit Kapila <amit.kapila16@gmail.com>
Debugged-By: Tom Lane <tgl@sss.pgh.pa.us>
Debugged-By: Ashutosh Sharma <ashu.coek88@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20201014203103.72oke6hqywcyhx7s@alap3.anarazel.de
Discussion: https://postgr.es/m/20201015083735.derdzysdtqdvxshp@alap3.anarazel.de