QThreadPool: optimize enqueueTask() for common case

the most-common case is: queue is empty or filled with tasks of
the same priority; so the runnable would be put at the end of queue.
both checks are cheap for us.

also avoid detach()'ing by using const iterators

Change-Id: Iab2255f852211f9accc8d717f778671661210ef3
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>