Rework introspection of table indexes and foreign keys on Postgres

[morozov]

Q	A
Type	bug

Fixes #5575.

The primary changes are in the PostgreSQL schema manager where instead of using ::REGCLASS to get the table name, we join the pg_class relation and get the name from there.

Additionally, there are some changes about quoting identifiers in order to make the second part of the test pass across platforms: on the one hand, we need to pass the table name quoted (otherwise, the Oracle's schema manager will upper-case it before introspection); on the other hand, the abstract schema manager had to be tweaked for that to work across the platforms.

PostgreSQLSchemaManagerTest::testListQuotedTable() has been repurposed since it covered a subset of what the new test covers and only for PostgreSQL.