Use cb_bucket_disconnect() to avoid duplication

Change-Id: I0524181c80b93cbfcf00b30d917cbba5bf8213fd
Reviewed-on: http://review.couchbase.org/27638
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>