common/Finisher: avoid memory re-allocations for finisher queue

common/Finisher: avoid memory re-allocations for finisher queue

Since there is only a single thread associated with the Finisher,
swap between two queues to avoid the need to re-allocate a new
vector for each iteration through the loop. Also replace the
condition broadcast with a signal since there is only a single
thread to wake up.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>