feat: in memory store implementation

[ramfox]

This is the first "pass" at adapting our code to better fit the DeltaChat/iroh-share use case.

Once we have an in-mem store implementation we can remove the RocksDB dependency. This dependency (and how long it takes to build) was a major blocker in our goal to get iroh-share into DeltaChat.

This feat should be followed up by #18