ReleaseUid: accept an integer as uid

We can't guarantee to keep the LockedObject around, when ReleaseUid is
called (because that might happen, for example, after a ganeti
shutdown/restart). So we'll support releasing on just an integer uid.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>