R1:966db4d38ec8

Add an intracluster synchronization log for cluster repositories

Summary:
Depends on D19778. Ref T13216. See PHI943, PHI889, et al.

We currently have a push log and a pull log, but do not separately log intracluster synchronization events. We've encountered several specific cases where having this kind of log would be helpful:

- In PHI943, an install was accidentally aborting locks early. Having timing information in the sync log would let us identify this more quickly.
- In PHI889, an install hit…