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

scripts/ssh/ssh-exec.php

@@ -307,7 +307,7 @@ try {
 $ssh_log->setData(
   array(
     'c' => $err,
-    'T' => (int)(1000000 * (microtime(true) - $ssh_start_time)),
+    'T' => phutil_microseconds_since($ssh_start_time),
   ));
 
 exit($err);