Only send TOPIC updates to a channel when the topic actually changed

Only send TOPIC updates to a channel when the topic actually changed

This prevents the channel from becoming flooded by unecessary TOPIC
update messages, that can happen when IRC services try to enforce a
certain topic but which is already set (at least on the local server),
for example. Therefore still forward it to all servers, but don't inform
local clients (still update setter and timestamp information, though!)