Limit the number of packet data blocks to 50, as packets are already limited

This is enough to prevent the fast back and forth of allocation and release
the pool is meant to prevent, but not too much so we don't hog memory

OpenSim/Framework/PacketPool.cs

@@ -234,6 +234,7 @@ namespace OpenSim.Framework
 
             lock (DataBlocks)
             {
+                if (DataBlocks[typeof(T)].Count < 50)
                     DataBlocks[typeof(T)].Push(block);
             }
         }