VoteCounters can shutdown after the vote completes

[Chris-Hibbert]

What is the Problem Being Solved?

VoteCounter vats hang around forever, in case anyone needs the outcome later. Recent discussions have clarified that we don't expect any consumers of this info after the fact on the live chain. Anyone that needs to review past votes can dredge it up from vstorage. (see #9372 for a summary)

Description of the Design

voteCounters can call zcf.shutdown() after sending out notifications about the outcome.

Security Considerations

None. Keeping the voteCounters wasn't due to a safety concern.

Scaling Considerations

Reduces disk usage for validators.

Test Plan

TBD

Upgrade Considerations

can accompany other proposals.