Implement accountsdb for engine account storage

[bmuddha]

Goal

Implement the accountsdb crate that routes and stores accounts for the engine.

Why

The engine needs a crate that owns account routing, persisted storage, and volatile storage for the mutable/immutable account model.

Scope

• implement accountsdb as the storage and routing layer for engine accounts
• wire persisted and volatile backends
• keep the crate aligned with the engine account model and storage layout