Instead of running GetDiff using the old diff cache and sanity checking
the results using the new diff cache, we switch the implementation to
use the new diff cache, and temporariy keep running the old diff cache
async to export metrics.
The new logic in this change is still gated with a config (false as
default) to rollout the new diff cache gradually on selective hosts. I
also removed the use-new-diff-cache request parameter as this is not
needed anymore (was used for debugging).
Change-Id: I34e10b2165bf0669c736ab17ebc2a871ccfbb8bd
02ac16d7af