query_supervisor: add option 'limits_enabled' instead of 'limited_users' list

The new option can be set for any role with ALTER ROLE or SET statements.
When a role got the option set to 'true' then all its queries are
limited by the supervisor.

Changes in functional test: use the new options instead of 'limted_users'
ALTER ROLE during tests must be used with ;IN DATABASE' to avoid change
global postrges state.

query supervisor: remove option limited_users