Priority processing of cosmic-swingset actions

[mhofman]

What is the Problem Being Solved?

We need to enable the priority processing of economic messages (#5334), in the absence of internal SwingSet priority queues (#4318), while avoiding contention with low priority messages that may already be queued and/or still be accepted by the network.

Description of the Design

We already process inbound actions one at a time, and run the kernel to completion before processing the next action (with the exception of timer polls).

#6576 will introduce a new priority inbound queue that is separate from the existing actionQueue.

Cosmic-swingset must then process items from the priority queue before any items present in the action queue.
Any work pending from the previous block would still be finished first (it does not preempt ongoing work, especially because that work may already be a high priority action).

It's currently undecided whether timer messages should take priority over priority messages (see #6964)

Security Considerations

None

Test Plan

TBD

[warner]

cc @gibson042

[mhofman]

Updated to reflect that we have a backpressure mechanism, that inbound/action queue entries are processed one at a time, and that the these are now staying in a vstorage backed queue.