Backend traits for Persistence layer

[vertexclique]

Is your feature request related to a problem? Please describe.
I want to store the state of the current mailbox and restore it when I need it.

Describe the solution you'd like
The persistence layer should have traits and also impl of possible external storage systems like rocksdb.

Describe alternatives you've considered
–

[vertexclique]

This is linked to #214 , if typed structs are passed as a handler. This will enable storing data in these structures with user's will. If that's not a feasible solution, we might need to seek for other alternatives for storing state individually for actors. But I think that will solve the problem.

cc @o0Ignition0o