Re: Reducing connection overhead in pg_upgrade compat check phase

Attached is a v10 rebase of this patch which had undergone significant bitrot
due to recent changes in the pg_upgrade check phase. This brings in the
changes into the proposed structure without changes to queries, with no
additional changes to the proposed functionality.

Testing with a completely empty v11 cluster fresh from initdb as the old
cluster shows a significant speedup (averaged over multiple runs, adjusted for
outliers):

patched: 53.59ms (52.78ms, 52.49ms, 55.49ms)
master : 125.87ms (125.23 ms, 125.67ms, 126.67ms)

Using a similarly empty cluster from master as the old cluster shows a smaller
speedup, which is expected since many checks only run for older versions:

patched: 33.36ms (32.82ms, 33.78ms, 33.47ms)
master : 44.87ms (44.73ms, 44.90ms 44.99ms)

The latter case is still pretty interesting IMO since it can speed up testing
where every millisecond gained matters.

--
Daniel Gustafsson