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.

## Example situation

- __CI__ starts _taskset1_
- __CI__ wait for _taskset1_ completion (= persisted)
- __rrrspec__ completes _taskset1_
- __rrrspec__ puts _taskset1_ into persister queue
- __rrrspec persister__ persists _taskset1_
- __rrrspec persister__ starts calculation
- __CI__ completes build of _taskset1_
- __CI__ starts _taskset2_
- __rrrspec__ completes _taskset2_
- __rrrspec__ puts _taskset2_ into persister queue
- __rrrspec persister__ is still calculating invoked by _taskset1_ _(this takes so long)_
- __rrrspec persister__ completes calculation for _taskset1_
- __rrrspec persister__ persists _taskset2_
- __CI__ completes build of _taskset2_