sql: Implement pg_catalog.pg_database as a virtual table

[nvanbenschoten]

See https://www.postgresql.org/docs/9.6/static/catalog-pg-database.html.

We need to be careful here about the distinction Postgres makes between Schemas and Databases.

@mjibson I assigned you because you said you were working on this already in Gitter. Feel free to reassign if that isn't true.