Use multi-inserts for pg_enum

Use multi-inserts for pg_enum

This allows to insert at once all the enum values defined with a given
type into pg_enum, reducing the WAL produced by roughly 10%~.  pg_enum's
indexes are opened and closed now once rather than N times.  The number
of items to insert is known in advance, making this change
straight-forward, and would happen on a CREATE TYPE .. AS ENUM.

The amount of data inserted is capped at 64kB for each insert batch.
This is similar to commits 63110c6 and e3931d01, that worked on
different catalogs.

Reported-by: Ranier Vilela
Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi, Ranier Vilela
Discussion: https://postgr.es/m/Y3M5bovrkTQbAO4W@paquier.xyz