CBQE-1507: memory stats can be changed while application is running

fix for cbstats memory

Change-Id: I365af7e470a4abe058a4d83370725b64ff9f70f3
Reviewed-on: http://review.couchbase.org/28060
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>