queue targets instead of splitting them

instead of splitting the target list into smaller bunches
to distribute them among the worker processes, the targets
are queued and whenever a worker process becomes ready, the
next target from the queue will be processed.

tags: relevant
$EXECUTION_FAILED was mis-spelled

tags: relevant
empty line removed

tags: insignificant
nested signal handlers are not possible

it is not possible to nest an SIGALRM handler inside an executing SIGALRM
handler. thus, if SIGALRM was hit it is not possible to cleanup using
$worker->delete() and setting up another alarm().

tags: relevant