[bundle] helper-mux feature

Hi all,
  for some reason I can't understand I can't manage to push this
feature-branch to launchpad.
It's a perl helper multiplexer: it talks the multi-slot helper dialect
to squid, and the single-slot variant to the helpers, starting them up
lazily and handling possible helper crashes.
Since squid aggressively tries to reuse the some helpers, setting a
high helpers number in conjunction with this has the effect of
allowing on-demand-startup of helpers.

I'm not sure it's production-ready, but it certainly is review-ready.

-- 
    /kinkie