Prompted by #386 (comment)

Currently Pow.Plug.delete_user/2 calls do_delete/2 for the plug, which only deletes the current session. However it should clear all sessions for the user. Only Pow.Store.CredentialsCache knows about related sessions which makes this tricky. Might work if a config value is passed on e.g. delete_all_for: user.

PowPersistentSession might interfere with this. The way to resolve that is to delete all persistent session tokens that has a session fingerprint that's deleted. There could be a method in PowPersistentSession.Store.PersistentSessionCache to do that, but it might also be a good idea to be able to look up all persistent sessions for the user.