Return value will be an associative array in [key => value] format. Keys
missing in cache will be omitted from the array.
Optionally, an 2nd variable can be passed to this function. It will be
filled with values that can be used for cas(), in an associative array in
[key => token] format. Keys missing in cache will be omitted from the
array.
getMulti is preferred over multiple individual get operations as you'll
get them all in 1 request.