CLEANUP: atomic: add an explicit _FETCH variant for add/sub/and/or

CLEANUP: atomic: add an explicit _FETCH variant for add/sub/and/or

Currently our atomic ops return a value but it's never known whether
the fetch is done before or after the operation, which causes some
confusion each time the value is desired. Let's create an explicit
variant of these operations suffixed with _FETCH to explicitly mention
that the fetch occurs after the operation, and make use of it at the
few call places.