Make ns_config keep a UUID representing config instance.

That's generated on first start-up and when config gets cleared.

Change-Id: Ia17418d9859fbdc1754c9604e18b7d2b59e1b146
Reviewed-on: http://review.couchbase.org/35292
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>