Revert "Create TimerStat, based on DistributionStat"

This reverts commit f7485a6a4067e0c0a36c2693cf9558eadd93ca9a which
was accidentally merged instead of a similar commit
2f1100a058040438e30e4cc0f90f0db1ba091105
Create TimeStat, based on QuantileDigest
Adjust tests for stats object changes