Add OS thread suspension

[msimberg]

This adds functionality to suspend and resume threads (and fixes throttling, thus replaces #2994).

Notes:

• in order to support suspending the pu on which the suspending thread is running the scheduling loop exit criteria have been relaxed a bit
• cleanup_terminated only cleans up terminated threads and returns true if there are none left (the checks for remaining work have been moved out to the scheduling loop)
• this adds a background thread count, the count is used to wait for the runtime/thread_pool_executor/this_thread_executor to finish their work (instead of relying on each pu to wait for other pus in the pool)
• {remove,add,suspend,resume}_processing_unit and create_thread (in schedulers) are locked with thread specific mutexes to avoid scheduling work when a pu is being suspended/removed (but the lock in create_thread is only taken if elasticity is enabled)
• create_thread checks the state of the scheduler to decide if it's running or not instead of checking used_processing_units (but again only if elasticity is enabled)
• the used_processing_units mask is only used for setting the affinity of service threads
• the used_processing_units mask is recomputed when requested to account for removed/suspended pus and different affinity masks per pu

Some schedulers are not tested:

• the static schedulers are not tested because they do not steal work
• the periodic scheduler is not tested because it doesn't check the state of the pus and might schedule work on suspended pus (candidate for removal, so I have not fixed this)

[msimberg]

This breaks some tests again with the latest commit, but I think I know the cause. Will try to fix it tomorrow.

[sithhell]

Works as advertised!