Switch TimedStat implementation to QuantileDigest

Some questions about the direction you're taking stats:

1) I don't see the point of having some stats use ExponentiallyDecayingSample and others use QuantileDigest. Is the intention to deprecate ExponentiallyDecayingSample?

2) QuantileDigest doesn't support the "return NaN in initial state" semantics expected by the TimedStat unit test. Was this an oversight or was it intentional?

3) MeterStat was deprecated in favor of DistributionStat, yet the latter doesn't support quantiles and the http client and server code still uses MeterStat. Would it not make more sense to un-deprecate MeterStat and reimplement it in terms of QuantileDigest?