feature: Allow running without persistent storage

[ibotty]

What is the motivation or use case for the feature?

I don't need persistence for my use case. Restarting the whole keydb cluster can start without any data, but when a cluster node goes down (because of e.g. hardware failure) it should start as soon as possible and get the current data as soon as possible. This works better without persistent volumes.

Describe the solution you'd like Using emptyDir storage instead.