Add/trial a `FoundationDB` WorldState backend

[rdaum]

Similar to #21 , I would like to see how this would perform and work out in code. I played with this in an earlier sketch of things, but didn't go far with it. In theory everything that is done via rocks can be done with FDB, but with distribution baked in (at the cost of performance and complexity)

They share a similar limitation though in that they're both optimized for a single-writer (per relation) but multiple reader workflow. Which may not describe the characteristics of what a transaction load is actually like in a working system.