Timer percentiles are not persisted like the base metric

[tiedotguy]

If no timer metrics are received in a flush interval for a timer metric persisted from the previous flush interval, the aggregator never calculates new percentiles (even as 0).

Depending on the behavior of the backend, this can cause invalid values where metric.upper_pct > metric.upper.

https://github.com/atlassian/gostatsd/blob/master/pkg/statsd/aggregator.go#L158 should probably create dummy values.

[tiedotguy]

Similar behavior when numInThreshold=0