This option exists to handle a race condition that can only occur when
there are concurrent writes to the same document. This doesn't apply
during offline reindex, and turning it off will speed things up.
It is not especially feasible to pass this option into ChangeIndexer in
code, so we have to use a config value for it. Remove the "test" prefix
from the config name, and document it.
Change-Id: Iecc12c3ab0f068f24063c358ce50a40b25362511
3747f0f7d4