Only delete empty expeditions when the dz is empty

Zones are no longer able to delete expeditions. World now tracks empty
expeditions in cache and only deletes them when it detects an
expedition's dynamic zone instance has no more clients inside.

This fixes an exploit where lockouts couldn't be applied to expeditions
after all members were removed because zones were deleting the expedition
immediately. Clients still inside the dz were able to complete events
before being kicked from the instance while not having an expedition.

Expeditions are no longer purged from database in the world purge
instance timer to avoid a possible race with this new system

world/main.cpp

@@ -607,7 +607,6 @@ int main(int argc, char** argv) {
 		if (PurgeInstanceTimer.Check()) {
 			database.PurgeExpiredInstances();
 			database.PurgeAllDeletedDataBuckets();
-			ExpeditionDatabase::PurgeExpiredExpeditions();
 			ExpeditionDatabase::PurgeExpiredCharacterLockouts();
 		}