MB-5177: log system stats periodically

Change-Id: I7b8f44e1b9ccbf5b6147459dbce0fa20b57d3614
Reviewed-on: http://review.couchbase.org/30125
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>