Use new vbucket_what_changed()

More accurate, consistent, and efficient managing of config updates.

Change-Id: I08f3a095a793385a9fc53286b511770c05539b17
Reviewed-on: http://review.couchbase.org/33802
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>