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_state.h

@@ -34,6 +34,7 @@ class ExpeditionState
 {
 public:
 	void AddExpedition(uint32_t expedition_id);
+	Expedition* GetExpedition(uint32_t expedition_id);
 	void LoadActiveExpeditions();
 	void MemberChange(uint32_t expedition_id, uint32_t character_id, bool remove);
 	void Process();