Re: optimizing pg_upgrade's once-in-each-database steps

As I mentioned elsewhere [0], here's a first attempt at parallelizing the
data type checks.  I was worried that I might have to refactor Daniel's
work in commit 347758b quite significantly, but I was able to avoid that by
using a set of generic callbacks and providing each task step an index to
the data_types_usage_checks array.

[0] https://postgr.es/m/Zov5kHbEyDMuHJI_%40nathan

-- 
nathan