Collocate update_vbucket_state and delete_vbucket handlers.

They are closely related, so it makes it easier to reason about
things.

Change-Id: I25dd1dbdfb6aa54e405e12bc56bf8e0f40efe78f
Reviewed-on: http://review.couchbase.org/35927
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>