Handle PostgreSQL enums in remote schemas

Handle PostgreSQL enums in remote schemas

Fixed issue where a :class:`.postgresql.ENUM` or a custom domain present
in a remote schema would not be recognized within column reflection if
the name of the enum/domain or the name of the schema required quoting.
A new parsing scheme now fully parses out quoted or non-quoted tokens
including support for SQL-escaped quotes.

Fixed issue where multiple :class:`.postgresql.ENUM` objects referred to
by the same :class:`.MetaData` object would fail to be created if
multiple objects had the same name under different schema names.  The
internal memoization the Postgresql dialect uses to track if it has
created a particular :class:`.postgresql.ENUM` in the database during
a DDL creation sequence now takes schema name into account.

Fixes: #4416
Change-Id: I8cf03069e10b12f409e9b6796e24fc5850979955