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.

Edit: a non cosmetic changes made it into the CL:

* Deduplication of visited keys and paths is always on. I encountered a run where the `list` command entered a cycle while traversing the shopify bucket (can happen as the bucket is undergoing concurrent changes while I traverse it).

@fbogsany 
I'm not aware of such a method..