MB-767: add config for incr. index measurements

Change-Id: I8c1ee0dfe3a8c2dee3cf3c905873db33d8d1e321
Reviewed-on: http://review.couchbase.org/24153
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>