Currently, we have `real_db.tables` and `schemas`, the former containing
system tables needed to parse statements, and the latter accumulating
user tables parsed from CQL. This will be error-prone to maintain with
view/index support, so ditch `schemas` and instead add a `user` flag to
`table` and accumulate all tables in `real_db.tables`.
At the end, just return the schemas of all user tables.