For now, the UpdateService ignores these fields when updating from
the index. There is no time that the index should specify what
versions to be ignored.
In the future, this will be done with a join table that stores
info about what to ignore. Another future improvement should also be
to make "App.toContentValues()" smarter. That is, make it only return
values which have been set since the object was created. However this
will add an overhead which may or may not be noticable.
