handle non-legacy mode stat lines from statsd.

* Move the shifting stuff into Util.
* Overhaul `MetricsWriterIntegrationTest` and `StatParserTest` to operate on stat lines in legacy/non-legacy prefix/non-prefix modes.
* I made it a little more obvious that `Stat.name` only applies to `Timer`s.
* Busted `Label` out of `Stat` and into its very own `StatLabel`
* `StatParser` was disbanded and reimplimented in `StatLabel` as `Parser` and `LegacyParser`.
* `StatsdOptions` abstraction captures key settings and defaults from `Configuration` and is used to conditionally parse stat lines.
* `StatsdConfig` contains statsd-specific configuration settings.