Identity services should be able to revoke all their keys on a whim

Currently they can't because `/isvalid` needs to say a key is valid forever if it is indeed valid, otherwise homeservers may unintentionally reject 3rd party invite events.

Some discussion is here: https://github.com/matrix-org/matrix-doc/pull/1505

This is probably a room v2 thing.

Start by reading the /isvalid requirement and the linked matrix-doc PR #1505; examine how the proposed room v2 behavior could support identity-service key revocation without causing homeservers to reject third-party invites. The issue names no files or tests, and its completion criteria remain to be defined through protocol design.

Written by the indexing model from the issue text.