Add option to turn off all fields in rescore

This is for a performance experiment - it doesn't change the default behavior.

Change-Id: I3f51e2a46b3a75f8380690b86ad160f617f19dfd
(cherry picked from commit 4a8be57238e202029ffede0f4260e9e34e436574)
Merge "Add option to turn off all fields in rescore" into wmf/1.24wmf20