+parameters: channel, topicTS, opt. topic setter, topic
+
+Propagates a channel topic as part of a burst.
+
+If the channel had no topic yet or the topicTS in the message is older than
+the topicTS of the current topic on the channel and the topics differ, set
+the topic with topicTS and topic setter, and propagate the message. Otherwise
+ignore the message and do not propagate it.
+
+If the topic setter is not present, use a server name instead.