Cleanup required for resource partitioner

[shoshijak]

• [x] Merge threadmanager and threadmanager_impl (since threadmanager is not templated on the scheduler any more, this type erasure mechanism is not needed any more) (see https://github.com/STEllAR-GROUP/hpx/tree/resource_partitioner_jb)
• [ ] Merge runtime and runtime_impl (same reasoning)
• [ ] Namespace review: move thread_pool and thread_pool_impl out of the detail namespace, and move threadmanager (and runtime?) into the detail namespace
• [ ] Scheduler cleanup: reduce the number of schedulers but make the remaining schedulers more flexible, for example, by turning flags on/off. Flags could be things like "stealing/none", "differentiation between priorities/not", "numa sensitive/not" etc.
• [x] Redundancy: both runtime and resource_partitioner have a pointer to the thread manager