Add forgetting methods to replica

[sgwilym]

What's the problem you want solved?

While the problem of distributed deletes is a hard problem, it should not be hard to make sure that data is deleted in its entirety from devices you control, temporarily.

This could be useful for situations where physical access to a user's device is at stake, e.g. a border crossing.

Is there a solution you'd like to recommend?

• Add a forgetDocuments method (which would take a Query)
• Add a forgetAttachment method (which would take a document)

[AljoschaMeyer]

Subsumes #18?