Change method dispatch in ClientOps to enforce luxi.REQ_ALL

Change method dispatch in ClientOps to enforce luxi.REQ_ALL

ClientOps' handle_request dispatches on the luxi request received. Change
this to first verify if the request is luxi.REQ_ALL. In this way, we catch
programming errors introducing "secret" luxi requests earlier. This is relevant,
as our other checks for internal consistency compare against luxi.REQ_ALL.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>