CBD-767: don't change ddoc during incremental index phase

Change-Id: Ie0399c2b9f00bd9d60d719a57c9de0894bd2f14a
Reviewed-on: http://review.couchbase.org/24166
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>