Use time.Since for cleaner (and more accurate) duration handling
Use time.Since for cleaner (and more accurate) duration handling

Summary:
Makes our log messages slightly more accurate (I think the diff before 0.9s and 1ms is relevant for debugging),
and cleaner overall.

Test Plan: Changes

Reviewers: jukka

Reviewed By: jukka

Subscribers: changesbot, mkedia, anupc

Differential Revision: https://tails.corp.dropbox.com/D130356

dropbox/changes-client,dropbox/changes-client