Add high water/ low water mark functionality for throttling jobs being
submitted.  So, if number of jobs is larger than high, job() will block until
number of jobs is lower than low.

Add Thread::Pool::LogResolve functionality.  Possibly add an IO-layer for
it.  Think about a solution for the problem

Add test-suite for "abort" functionality.

Add test-suite for submitting jobs from different threads to the same
Thread::Pool object.  Important for use in the worker MPM of Apache 2.0.

Add examples.

Add automatic handling of adding/removing threads a la Apache.
