R1:6e03419593a6

Implement a rough AlmanacService blueprint in Drydock

Summary:
Ref T9253. Broadly, this realigns Allocator behavior to be more consistent and straightforward and amenable to intended future changes.

This attempts to make language more consistent: resources are "allocated" and leases are "acquired".

This prepares for (but does not implement) optimistic "slot locking", as discussed in D10304. Although I suspect some blueprints will need to perform other locking eventually, this does feel like a good fit…