Don't create an items list in set_many.

There's another performance note here anyhow, but this at least removes the intermediate list from `items()`.

Also, makes the docstring not lie :D, since before it only returned `True` for empty values, not for anytime the sets all succeeded.

(Though TBH unless there's something more complicated planned or this is some compatibility with other memcache API thing I'd just have had this and set return `None` [i.e. do nothing special] if it were me. Exceptions are good enough signals that something's up).