Fix duplicate cycles

[jwright]

This PR fixes an issue where we would get duplicate current usage trackers.

This was caused by having multiple verbs with the same limit cycles on them.

# products.yml

basic:
  limits:
    foos:
      count: 10
      enforcement: hard
      interval: month
      duration: 1
    bars:
      count: 10
      enforcement: hard
      interval: month
      duration: 1

The Billing::Usage::Tracker.cycles would return duplicate 1 month intervals and that would cause HasTrackers.billing_usage_trackers.current to return duplicate trackers for the single cycle.

Closes #12