ChanServ: add SET SUCCESSOR with founder inheritance

SET <#channel> SUCCESSOR <account>|OFF names an account that inherits the
channel if the founder's account is dropped or expires, instead of the
channel being released. Adds a ChannelInfo.successor field + event, and
centralises the founder-gone channel release into a single
release_founded_channels store method used by DROP, expiry, and the
gossip account-gone path so all three transfer consistently.

src/grpc.rs

@@ -145,6 +145,7 @@ fn to_wire(entry: &LogEntry) -> Option<ReplicationEvent> {
         | Event::ChannelAkickAdd { .. }
         | Event::ChannelAkickDel { .. }
         | Event::ChannelEntryMsgSet { .. }
+        | Event::ChannelSuccessorSet { .. }
         | Event::ChannelSettingsSet { .. }
         | Event::ChannelKickerSet { .. }
         | Event::ChannelBadwordsSet { .. }