We now explicitly track *committed* assertions of each actor in a new
field, `actor-cleanup-changes`. Each time a patch action is
*performed*, `actor-cleanup-changes` is updated. When an actor quits,
it enqueues a special new kind of action, a `quit` action.
When a `quit` action is performed, any remaining contents of
`actor-cleanup-changes` are processed in order to fully remove any
leftover assertions. (Leftover assertions will only arise in
exceptional cases: when some stop-script or facet boot-script raises
an uncaught exception.)
As part of this commit, we undo the effect of commit b207a07.
Tony Garnock-Jones