Make redshift a "first-class" fixture

[DanCardin]

Is your feature request related to a problem? Please describe.
Today redshift just piggybacks off postgres's container definition to start its container.

This precludes us from making database-wide changes, like SET session_replication_role = 'replica'; which is a potential option for #62 and other specific kinds of tests.

If we used the same existing container startup mechanisms for redshift as we do for postgres, but left the host/port/user/pass the same; then PMR will think it has already been started. And then we'll either want to prefix the database name with "postgres/redshift" or ensure they're both using the same PMR-specific table for record-keeping on tests.

With that said, we'd have the same default behavior as today, but with the ability to theoretically configure the settings to be different for redshift and produce a different container.

[DanCardin]

Closed by #138