Implement a global resource-control for WMAgent work acquisition

[amaltaro]

Impact of the new feature
WMAgent

Is your feature request related to a problem? Please describe.
Each agent has its own database and resource control, making it sub-optimal to control how much work the agents have allocated / pending for each site.

Describe the solution you'd like
I would suggest to make a central resource-control database, such that agents can consult it before pulling work from central workqueue, and also update it once work has been materialized into jobs.
It's basically the same scenario we have right now, but instead of having one resource control per agent, we would have a single place which would give an overview for the whole central production system.
It would be a single place for setting sites to enabled/disabled/drain as well.

Of course, REST APIs should be properly written to read/write to this unique resource control.

Describe alternatives you've considered
Keep using the current mechanism

Additional context
None