Simplify our APIs and force dwaitpid() to work in async mode for
all lei workers. This avoids having lingering zombies for
parallel searches if one worker finishes soon before another.
The old distinction between "old" and "new" workers was
needlessly complex, error-prone, and embarrasingly bad.
We also never handled v2:// writers properly before on
Ctrl-C/Ctrl-Z (SIGINT/SIGTSTP), so add them to @WQ_KEYS
to ensure they get handled by $lei when appropropriate.