use actual ephemeral memory instead of time-invalidated storage

[tomholub]

I don't think this implements storage in memory at all - it stores pass phrases in the database, and invalidates them on retrieval if they are too old. This is not what I wanted, unless I overlooked something.

Originally posted by @tomholub in #323 (comment)

The pass phrases should be kept either in storage permanently, or in RAM up to 4 hours depending on user settings.