pgsql: Set cutoff xmin more aggressively when vacuuming a temporary tab

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a7212be8b9e0885ee769e8c55f99ef742cda487b

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c |  1 +
src/backend/commands/cluster.c       | 28 ++++++++------
src/backend/commands/vacuum.c        | 73 ++++++++++++++++++++++++------------
src/include/commands/cluster.h       |  3 +-
src/include/commands/vacuum.h        |  1 +
5 files changed, 70 insertions(+), 36 deletions(-)