bbf72e7 made a change where you can pass `false` to certain MemChecks
functions to get them to skip performing an "update" step. It was then
up to the caller to call the Update function later.
This commit changes the implementation so that, instead of the caller
passing in a boolean that controls whether a function calls Update, the
function now returns an object that on destruction will call Update.
Callers that are fine with Update being called right away can skip
storing the object in a variable and thereby call Update immediately,
and callers that want to call Update later can keep the object around.
This new design reduces the risk that someone will forget calling
Update.
303366b1ce