Erase connection windows before discarding them.

And I rewrote the locking in ConnectionRecogniserImpl again. I hate
that class so much.

api/net/sf/briar/api/transport/ConnectionWindow.java

@@ -9,4 +9,6 @@ public interface ConnectionWindow {
 	byte[] setSeen(long connection);
 
 	Map<Long, byte[]> getUnseen();
+
+	void erase();
 }