This removes the config and code to output to statsd.
- Change `cmd.StatsAndLogging` to output a `Scope`, not a `Statter`.
- Remove the prefixing of component name (e.g. "VA") in front of stats; this was stripped by `autoProm` but now no longer needs to be.
- Delete vendored statsd client.
- Delete `MockStatter` (generated by gomock) and `mocks.Statter` (hand generated) in favor of mocking `metrics.Scope`, which is the interface we now use everywhere.
- Remove a few unused methods on `metrics.Scope`, and update its generated mock.
- Refactor `autoProm` and add `autoRegisterer`, which can be included in a `metrics.Scope`, avoiding global state. `autoProm` now registers everything with the `prometheus.Registerer` it is given.
- Change va_test.go's `setup()` to not return a stats object; instead the individual tests that care about stats override `va.stats` directly.
Fixes#2639, #2733.
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