expiration of in-memory pass phrases should happen inside InMemoryPassPhraseStorage

[tomholub]

There should be a better separation of responsibilities. Everything relating to time tracking and expiring old in-memory passphrases should be handled in InMemoryPassPhraseStorage. This class that combines the two methods should be more dumb. I'll file a separate issue.

Originally posted by @tomholub in #323 (comment)