Make it possible to cancel packets for asynchronous processing.

ProtocolLib/src/main/java/com/comphenix/protocol/async/PacketSendingQueue.java

@@ -89,6 +89,7 @@ abstract class PacketSendingQueue {
 			PacketEvent copy = PacketEvent.fromSynchronous(packetUpdated, marker);
 			
 			// "Cancel" the original event
+			packetUpdated.setReadOnly(false);
 			packetUpdated.setCancelled(true);
 			
 			// Enqueue the copy with the new sending index

ProtocolLib/src/main/java/com/comphenix/protocol/injector/PacketFilterManager.java

@@ -496,6 +496,7 @@ public final class PacketFilterManager implements ProtocolManager, ListenerInvok
 			asyncFilterManager.enqueueSyncPacket(event, event.getAsyncMarker());
 
 			// The above makes a copy of the event, so it's safe to cancel it
+			event.setReadOnly(false);
 			event.setCancelled(true);
 		}
 	}