MB-11976: Wait to grab a more accurate memory usage

Change-Id: I62edb052a5e22b2bf9a8ec7a781258e78e58c802
Reviewed-on: http://review.couchbase.org/40797
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>