Implement DDL visitor for PG ENUM with schema translate support

Implement DDL visitor for PG ENUM with schema translate support

Fixed issue where the :class:`_postgresql.ENUM` type would not consult the
schema translate map when emitting a CREATE TYPE or DROP TYPE during the
test to see if the type exists or not.  Additionally, repaired an issue
where if the same enum were encountered multiple times in a single DDL
sequence, the "check" query would run repeatedly rather than relying upon a
cached value.

Fixes: #5520
Change-Id: I79f46e29ac0168e873ff178c242f8d78f6679aeb