Avg calulation shouldn't slow down persistence

Updating averages is too slow; yes, I know we should fix that...

The problem is, `rrrspec waitfor` command waits the calculation. Not only a taskset. Because it waits persistence.

Persistence phase may wait for previous average calculation. This makes a lag, so `rrrspec waitfor` may wait for previous calulation.

This extract statistics calculation to the another thread, and allow Persister to work persisting of another completed taskset faster.