MB-14762 Fix memory leak in system_stats_collector.
MB-14762 Fix memory leak in system_stats_collector.

It would accumulate an unbounded list of per-process stats in
PrevSample.

Change-Id: Ibf204d945933a75e27a831392b10ba51787dbc5c
Reviewed-on: http://review.couchbase.org/50433
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Poonam D <6855f09986fe28ed7e8cdd9d60db878d898703d5@couchbase.com>

nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server