QuickJS: discard contexts with pending jobs

QuickJS: discard contexts with pending jobs

A job exception stops the teardown drain immediately.  Jobs left behind
could then run after the context was bound to another request.  Releasing
events and root values after the drain could also enqueue new jobs with
the same result.

Treat a successful, complete job drain as a precondition for context
reuse.  Destroy the runtime when the drain fails or the queue becomes
nonempty during the remaining teardown.