MB-10472: Simplify ChangesWrapper function

Unify the ChangesWrapper function for the initial and incremental
index build.

Change-Id: I17910f8934c5353f6011ddb597d280096341349e
Reviewed-on: http://review.couchbase.org/37178
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>