Throttle auto expedition leader changes (#1293)

This fixes unnecessary leader processing on mass dzquits

Also marks leader dirty for empty expeditions in case something goes
wrong (shouldn't be possible after 62e480fe)

world/expedition_state.cpp

@@ -145,6 +145,7 @@ void ExpeditionState::Process()
 		else
 		{
 			it->CheckExpireWarning();
+			it->CheckLeader();
 		}
 
 		it = is_deleted ? m_expeditions.erase(it) : it + 1;