low latency threads: Add this abstraction

Low latency threads are threads that are initialized with high
priority.

Also added an algorithm that updates the priority according to the
current load level:
   - If load level is too high ("throughput" workload)- decrease the
     priority in order to ensure fairness between a low latency thread and other threads.
   - If load level is too low ("latency" workload) - increase the priority
     to ensure the best response time.

The algorithm above will check the average load level every 100ms.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>