Protected Hired Merchant + Buff system patch

Fixed some issues with Fredrick not retrieving the right amount of items
from Hired Merchants. Added concurrency protection for HM. Patched a
minor issue on buff system.

src/net/server/world/World.java

@@ -86,7 +86,6 @@ public class World {
     private long mountUpdate;
     
     private Map<HiredMerchant, Byte> activeMerchants = new LinkedHashMap<>();
-    private ScheduledFuture<?> MerchantsSchedule;
     private long merchantUpdate;
     
     private ScheduledFuture<?> charactersSchedule;