MB-100 change set_vbucket_state from call to cast

because apparently cast doesn't cause race

Change-Id: I34722aae9bc1b4f95ee2be00c86ba205f759cf0d
Reviewed-on: http://review.couchbase.org/34916
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>