#3416のBP#3497のコミットを3件分

revoke automatic login cookie before registration (fixes #3497 BP from #3416)
(cherry picked from commit 2cac338f0edd68b57d534413e54d4b4380237a83)

use empty string to clear member cache (fixes #3497 BP from #3416)
(cherry picked from commit eb5fe39f5619e7bcac973ca61bd32a94bfa28090)

revoke automatic login cookie on registration pages using register_token (fixes #3497 BP from #3416) 
(cherry picked from commit 8fbdf778cd6133103bbf08a261928893a8c2eb0d)