statistics: insert new bin into histogram with move semantics.
statistics: reset histogram bin's timestamp if there's no data.
chrono: time_unit_from_string(), time_unit_as_string() added.
config: configuration format simplified:
- "statistics" -> "defaults"
- "core.enable" -> "enable"
- "tags" -> "stats"
- "metrics-dump.interval" -> "dump-interval"