We need a way to be able to expire data from a HS. (SPEC-141)

This could be implementation-specific, or via a standard admin API or per-room state configuration.

Eitherway, we need a way to tell HSes to:
- no longer store this room
- only keep N messages for this room
- expire more than N messages for this room
- only keep X days of messages for this room
- purge state from this room (as opposed to messages)?

(Imported from https://matrix.org/jira/browse/SPEC-141)

(Reported by @ara4n)

Start by reviewing the requirements in this issue and the imported SPEC-141 discussion on the linked Matrix JIRA page. Resolve whether expiration should be implementation-specific, exposed through an admin API, or configured per room, and define what each requested retention and purge operation means before considering the work complete.

Written by the indexing model from the issue text.