Ability to disable low priority processing

[mhofman]

What is the Problem Being Solved?

For recovery purposes (#4317) as well as for testing, we need a mechanism that completely disables the processing of low priority queues. In the first case, we want to make sure the chain can catch up to the real world after a halt. In the second case, we would be able to easily verify that no low priority vat lays in the reaction path of an external economic event.

Description of the Design

TBD

Security Considerations

The ability to pause low priority execution should only be held by the kernel / embedder. The ability to resume low priority execution may need to be delegated to economy handling vats, which would be more complicated.

Test Plan

TBD