Introduced kind of performance monitor.

Instead of high priority messages now there are low priority actors. These could be back-end jobs that share resources with front-end jobs.  Custom IFredisPerformanceMonitor could suspend execution of low priority actors upon some condition and Fredis will recheck this condition with a given frequency.

E.g. a web search engine could get high priority queries and run crawlers on the same boxes. If number of queries spikes and CPU usage grows above 80% per minute, crawlers could be suspended until CPU usage falls below 50%. This will give resources to high priority jobs. If then AWS autoscale group is set to grow when CPU usage exceeds 65% per 5 minutes, new boxes will be added only when there are too many high priority jobs.

It takes time (up to 20 minutes) to add new boxes, and it takes money to keep constantly low load. Better to keep constant load in 50-80% range and have quickly available capacity for important tasks.