Support for partial, exclusive allocation of pool resources

[jameshcorbett]

Fluxion has a limitation when it comes to pool resources. From the resource-query docs:

Please note that the granularity of exclusive allocation/reservation is the whole resource pool vertex, not anything less. Thus, if you want a more fined-grained exclusive memory allocation, for instance, you should first model your memory pool vertices with smaller memory unit (e.g., 256MB).

So our resource graph uses rabbit storage slices of customizable size. But to allow finer granularity of requests and to prevent an explosion of the resource graph, Fluxion will need to expand its functionality to allow allocations from resource pools with arbitrary granularity.

See also #722