Currently storage layout relies on a root node in the form of a Service Actor with separate storage collections for actors, objects and activities.

I would like to switch from this to a layout where the top level is just a list of actors, which contain collections, which contain activities and then objects.

This makes it easier for the storage engines that have per database locks (badger, sqlite, and maybe boltdb).

As an example for sqlite, each actor will have its own database and allow concurrent writes. :D

This is related to #281