pgsql: Cache the results of format_type() queries in pg_dump.

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

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9407dbbcb5b587cbefc4af14d1612b49abcb143b

Modified Files
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src/bin/pg_dump/pg_dump.c | 13 +++++++++++--
src/bin/pg_dump/pg_dump.h | 6 ++++--
2 files changed, 15 insertions(+), 4 deletions(-)