Apply partition changes to indexes immediately.

After indexes switched to UPR, there's no need to ensure that vbuckets
physically exist on disk.

Change-Id: I0eb1e9edcfbd941112763bc9cda93ebdc8bab383
Reviewed-on: http://review.couchbase.org/35701
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>