fix(swingset): add exit/vatstore syscalls to non-local vat workers

[warner]

Some unfortunate code duplication meant the nodeworker/subprocess-node
workers were lacking vatstoreGet, vatstoreSet, and vatstoreDelete. And
everything but the local worker was lacking syscall.exit.

refs #2643 , but doesn't close it because I want to (eventually) write proper
tests for this

cc @dckc just so you know this is getting addressed :)

[FUDCo]

Longer term, there's a lot of code that is replicated across the various vat worker implementations that probably should be refactored to only exist in one place.