change the TS, notifying local users of this but not servers (note that
ban-like modes remain intact; invites may or may not be cleared).
+A JOIN is propagated with the new TS of the channel.
parameters: channel, target user, opt. reason
split riding may happen: if the key (+k) differs or the incoming simple modes
include +i, kick all local users, sending KICK messages to servers.
+An SJOIN is propagated with the new TS and modes of the channel. The statuses
+are propagated if and only if they were accepted.
SJOIN must be used to propagate channel creation and in netbursts. For regular
users joining channels, JOIN must be used. Pseudoservers may use SJOIN to join