Allow kick_on_split_riding to protect channels with mlocked keys.

Allow kick_on_split_riding to protect channels with mlocked keys.

Do kick_on_split_riding if services sends an SJOIN
with a lower TS and a different key. This relies on
services restoring TS (changets option in atheme) and
services not immediately parting after receiving the
KICK, which is the case in recent atheme.

For invite-only channels, still only do
kick_on_split_riding in netbursts. Services is
assumed to handle this itself (atheme does).