H:A:Polite - permit more than one in_progress per domain

probably incomplete!
  interaction of delay-after-fetch and req_per_host is untested
  there are probably now holes in the POD
  no t/polite-multi.t

(this worked for me as a local monkey patch,
 just pushing in case it's useful)
H:A and H:A:P - new method clear_send_queue

I wired it up to $SIG{INT}

(it lacks a test, sorry)