Add VACUUM instrumentation for scanned pages, relfrozenxid.

Add VACUUM instrumentation for scanned pages, relfrozenxid.

Report on scanned pages within VACUUM VERBOSE and autovacuum logging.
These are pages that were physically examined during the VACUUM
operation.  Note that this can include a small number of pages that were
marked all-visible in the visibility map by some earlier VACUUM
operation.  VACUUM won't skip all-visible pages that aren't part of a
range of all-visible pages that's at least 32 blocks in length (partly
to avoid missing out on opportunities to advance relfrozenxid during
non-aggressive VACUUMs).

Commit 44fa8488 simplified the definition of scanned pages.  It became
the complement of the pages (of those pages from rel_pages) that were
skipped using the visibility map.  And so scanned pages precisely
indicates how effective the visibility map was at saving work.  (Before
now we displayed the number of pages skipped via the visibility map when
happened to be frozen pages, but not when they were merely all-visible,
which was less useful to users.)

Rename the user-visible OldestXmin output field to "removal cutoff", and
show some supplementary information: how far behind the cutoff is
(number of XIDs behind) by the time the VACUUM operation finished.  This
will help users to figure out what's _not_ working in extreme cases
where VACUUM is fundamentally unable to remove dead tuples or freeze
older tuples (e.g., due to a leaked replication slot).  Also report when
relfrozenxid is advanced by VACUUM in output that immediately follows
"removal cutoff".  This structure is intended to highlight the
relationship between the new relfrozenxid value for the table, and the
VACUUM operation's removal cutoff.

Finally, add instrumentation of "missed dead tuples", and the number of
pages that had at least one such tuple.  These are fully DEAD (not just
RECENTLY_DEAD) tuples with storage that could not be pruned due to
failure to acquire a cleanup lock on a heap page.  This is a replacement
for the "skipped due to pin" instrumentation removed by commit 44fa8488.
It shows more details than before for pages where failing to get a
cleanup lock actually resulted in VACUUM missing out on useful work, but
usually shows nothing at all instead (the mere fact that we couldn't get
a cleanup lock is usually of no consequence whatsoever now).

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wznp=c=Opj8Z7RMR3G=ec3_JfGYMN_YvmCEjoPCHzWbx0g@mail.gmail.com