Use phutil_microseconds_since(...) to simplify some timing arithmetic

Summary: Depends on D19796. Simplify some timing code by using phutil_microseconds_since() instead of duplicate casting and arithmetic.

Test Plan: Grepped for 1000000 to find these. Pulled, pushed, made a conduit call. This isn't exhaustive but it should be hard for these to break in a bad way since they're all just diagnostic.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19797