Let world handle expedition leader changes

This should eliminate race conditions caused by zones trying to set
a leader when members in different zones quit at the same time

Zone still detects when leader goes offline to trigger a change
since it's easier than having world process expedition member status
updates and perform expedition lookups

world/expedition_message.h

@@ -26,6 +26,7 @@ class ServerPacket;
 namespace ExpeditionMessage
 {
 	void AddPlayer(ServerPacket* pack);
+	void ChooseNewLeader(ServerPacket* pack);
 	void GetOnlineMembers(ServerPacket* pack);
 	void HandleZoneMessage(ServerPacket* pack);
 	void MakeLeader(ServerPacket* pack);