Run vbucket state changes in a separate worker process.

So stop rebalance works properly. Also, all the state changes are
executed in order.

Change-Id: I10a72c6c600aa1227ecc99e28139de58f7a55b37
Reviewed-on: http://review.couchbase.org/35928
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>