MB-100 rename mem_used @system stat to mem_used_sys

- because it conflicts with and overwrites mem_used bucket stat

Change-Id: I725e8ef95533d792b2ead5a3aca70c2a2ab06092
Reviewed-on: http://review.couchbase.org/29216
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>