Speed up introspection of system objects like Database and Role

[msullivan]

Currently if there are a lot of roles, doing a query like select sys::Role { member_of } is quite slow. This is because the views for
both Role and member_of need to extract metadata for every Role
and pase it as json, and postgres is planning this as a nested loop.

Unfortunately, startup needs to query all these objects, so having
lots of databases or roles is a problem.

Fix this by forcing sys objects and links to always be materialized
into CTEs. This ensures that we do at most one linear scan of each of
the views. Since there is no way to hit an index when querying any of
these system object views, always doing a linear scan is no worse.

For the objects themselves we can leverage the rewrites mechanism. For
link tables, we add a slightly lighter weight mechanism to handle it.

Tested using 100 roles.

Reverts the now-unnecessary main part of #6633 but keeps the extension
to the patch system. We could actually put annotations on databases now
and have it not be too slow.

This can be backported with an empty edgeql+schema+globalonly patch.

[aljazerzen]

This is a clean solution, I like it.