Provide way to not use disk to persist data at all, only in-memory

[raydac]

May be it would be a good idea to provide a way to start brokering only "in-memory" without persisting data. It could provide a better way to use Redpanda for tests and also build solutions where developers would like to use the tool but without strong persistence guarantees.
Just some configurable embedded in-memory store instead of disk on level of application with provided metrics about its use.