Cache the results of format_type() queries in pg_dump.

There's long been a "TODO: there might be some value in caching
the results" annotation on pg_dump's getFormattedTypeName function;
but we hadn't gotten around to checking what it was costing us to
repetitively look up type names.  It turns out that when dumping the
current regression database, about 10% of the total number of queries
issued are duplicative format_type() queries.  However, Hubert Depesz
Lubaczewski reported a not-unusual case where these account for over
half of the queries issued by pg_dump.  Individually these queries
aren't expensive, but when network lag is a factor, they add up to a
problem.  We can very easily add some caching to getFormattedTypeName
to solve it.

Since this is such a simple fix and can have a visible performance
benefit, back-patch to all supported branches.

Discussion: https://postgr.es/m/20210826084430.GA26282@depesz.com

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