forget(): Delete local data using a replication filter

[cinnamon-bun]

Related to #6 (Replication filters)

It should be possible to drop local content that matches or doesn't match a query. You would want to do this if you've just blocked someone, or if unfollowing someone means you want fewer people in your local database.

This isn't a delete message that will propagate, we're just forgetting local data.

• Add a forget(query) method to IStorage
• Do we also want the opposite, onlyKeep(query) ?