Make Postgres server a plugin

Same basic idea, but a little bit more complicated because of dependencies on Postgres in random places in core. In particular in tests. These need to be moved into the Postgres module or made to work some other way. Both approaches are taken here.

User visible renaming of some `fdbsql.postgres.` properties which become `fdbsql.sql.` so that they can be used by core or REST, too.