Maintain comms vat state in vatstore instead of memory

[FUDCo]

This is a follow-on to #2013

What is the Problem Being Solved?

Comms vat memory fills up in proportion to message traffic and the number of remote connections that have been active during its lifetime, so memory use can grow without bound. All state should be kept on disk so that active memory only reflects active use and the memory image size stays constant. (Since this stuff will still consume disk space, eventually there should be measures taken to clean up of old stuff that is no longer used, such as connections to dead vats; this would be part of the larger SwingSet garbage collection problem, but that is not this task.)

Description of the Design

Maintain all working state in the vatstore using essentially the same pattern that the kernel uses for keeping its state in the kernel DB.

Security Considerations

No new ones that I can think of.

Test Plan

The existing comms vat tests should be sufficient to capture tests for correctness. Our ongoing measurement and monitoring of the memory footprint of vats running our economy should help us gauge if this is addressing the comms vat portion of our ongoing memory issues (and if these measurements don't tell us enough to know then they should be augmented so that they do).

[FUDCo]

Closed by #2856