R1:df3c937dab8c

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.