Clean up old messages from other servers in active rooms

[MurzNN]

Will be good to add an option for cleanup old messages in active rooms, that sent from other Matrix servers, from local database. Eg all messages older than 3 months.

Those messages can be deleted and will be be synced automatically on demand from other Matrix severs (if owning server still alive). In large rooms like Matrix HQ this will greatly decrease room size on local storage database.

[MurzNN]

Seems here https://github.com/djmaze/synapse-purge is the script, that implement this feature, but I don't test it.