[issue #32] proper upper bound for cached limited thread pool

The cached limied thread pool is now initialised with a core thread
count of maxThreads. However, in order to not behave like an ordinary
fixed thread pool the amount of available threads grows on demand and
can shrink back to zero again (core threads may timeout, too).
Merge branch 'bug/32-max-threads-for-cached-limited-pool' into dev