…cy score

Problem: We never cleaned the known sessions, even if all of them had too low latency score to use; example of what sometimes happens:
1) Broadcaster accumulated 8 known sessions, but all had too low latency score
2) Session refresh was not triggered because we had a lot of knows session in the pool
3) At the same time, we didn't have sessions in unknown session pool
4) Selection checked that it cannot use the known session and tried to select from unknown sessions (but the pool was empty or there were no sessions fulfilling the condition of max price or perf score)