Record lock timing information on PushEvents

Summary:
Depends on D19249. Ref T13109. Add timing information to the PushEvent:

• writeWait: Time spent waiting for a write lock.
• readWait: Time spent waiting for a read lock.
• hostWait: Roughly, total time spent on the leaf node.

The primary goal here is to see if readWait is meaningful in the wild. If it is, that motivates smarter routing, and the value of smarter routing can be demonstrated by looking for a reduction in read wait times.

Test Plan: Pushed some stuff, saw reasonable timing values in the table. Saw timing information in "Export Data".

Maniphest Tasks: T13109

Differential Revision: https://secure.phabricator.com/D19250