Metrics and logging

This is mostly a cosmetic PR, it changes the logger to use `logrus` exported funcs. It also uses `expvar` for metrics, which the [upcoming Datadog agent (v5.0)](https://github.com/DataDog/dd-agent/blob/73dd75035f1982d31818bb8a4a2a6cc856cf8bb7/CHANGELOG.md#500--unreleased) will plug into out of the box.

@fbogsany 
Urh, right it's not cosmetic. 

This is because running without `dedup` failed once (infinite recursion of the shopify bucket), meaning it should not be a variable - it should always dedup.  Originally I had not encountered situations were dedup mattered, and made it variable to save memory, but leaving the other branch in the code. It proved a bad option, so removed.