Fix Phrequent duration accounting

Summary: Fixes T5705. This was just derp; instead of returning the duration of the first slice, return the duration of all the slices.

Test Plan: Added unit tests. Saw reasonable results in the UI.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

Subscribers: epriestley

Maniphest Tasks: T5705

Differential Revision: https://secure.phabricator.com/D10184