core: fix pthreads performance

OpenCV pthreads-based implementation changes:
- rework worker threads pool, allow to execute job by the main thread too
- rework synchronization scheme (wait for job completion, threads 'pong' answer is not required)
- allow "active wait" (spin) by worker threads and by the main thread
- use _mm_pause() during active wait (support for Hyper-Threading technology)
- use sched_yield() to avoid preemption of still working other workers
- don't use getTickCount()
- optional builtin thread pool profiler (disabled by compilation flag)

modules/core/include/opencv2/core/utility.hpp

@@ -233,6 +233,8 @@ CV_EXPORTS_W int getNumThreads();
 /** @brief Returns the index of the currently executed thread within the current parallel region. Always
 returns 0 if called outside of parallel region.
 
+@deprecated Current implementation doesn't corresponding to this documentation.
+
 The exact meaning of the return value depends on the threading framework used by OpenCV library:
 - `TBB` - Unsupported with current 4.1 TBB release. Maybe will be supported in future.
 - `OpenMP` - The thread number, within the current team, of the calling thread.