* jobspec: rename node pool scheduler_configuration
In HCL specifications we usually call configuration blocks `config`
instead of `configuration`.
* np: add memory oversubscription config
* np: make scheduler config ENT
Implementation of the base work for the new node pools feature. It includes a new `NodePool` struct and its corresponding state store table.
Upon start the state store is populated with two built-in node pools that cannot be modified nor deleted:
* `all` is a node pool that always includes all nodes in the cluster.
* `default` is the node pool where nodes that don't specify a node pool in their configuration are placed.
