R1:36e72639de66

Reduce visibility of "Host" and "Path" Differential fields by default

Summary:
See discussion in T838. These fields expose information which it isn't necessary
or useful to expose in the general case.

- Disable fields by default, allow them to be enabled in config (these fields
were useful for me at Facebook when I had access to all the machines).
- Remove 'sourcePath' from Conduit methods other than differential.query.
- Condition 'sourcePath' field in Conduit on the caller being the revision…