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

resources/sql/autopatches/20180322.lock.02.wait.sql

@@ -0,0 +1,8 @@
+ALTER TABLE {$NAMESPACE}_repository.repository_pushevent
+  ADD writeWait BIGINT UNSIGNED;
+
+ALTER TABLE {$NAMESPACE}_repository.repository_pushevent
+  ADD readWait BIGINT UNSIGNED;
+
+ALTER TABLE {$NAMESPACE}_repository.repository_pushevent
+  ADD hostWait BIGINT UNSIGNED;