Fixed size ThreadPool scheduler

[maierlars]

Scope & Purpose

This PR implements a new ThreadPoolScheduler with a fixed number of threads, and a number of different thread pools. The ThreadPoolScheduler creates a separate thread pool for each priority lane.

We have the following thread pools:

• SimpleThreadPool - this is a very simple thread pool with a std::queue, mutex + condition variable
• LockFreeThreadPool - this one is similar to the SimpleThreadPool but replaces the std::queue with a boost::lockfree::queue
• WorkStealingThreadPool - this is a first version of a work stealing pool where instead of a globally shared queue, each thread has a local queue

This PR also implements a number of synthetic benchmarks as tests (by default these are disabled so that they do not run durng regular builds). These benchmarks showed, that the WorkStealingThreadPool scales significantly better than the other two thread pools, and also much better then the current SupervisedScheduler.

[goedderz]

Soo much simpler 😍

[jsteemann]

LGTM