Enable idle rate in CMake if creation rate/cleanup rate is enabled in CMake.
Implement priority propagation for critical priority HPX-threads. Now, whenever an
HPX-thread with critical priority creates another HPX-thread, the child HPX-thread
will be set to critical as well. This significantly improves the regularity of
HPX performance counters evaluated at regular intervals (now those intervals are
actually... regular).

Hartmut, Heller: Should we only propagate critical priority if the created HPX-thread
is specified as having a "default" priority? We should talk about this before merging
this.