[GSS](7.0.z) UNICAST3 drops all messages until it receives the first one

UNICAST3.getReceiverEntry returns null if it hasn't seen the first message yet.
This causes UNICAST3.handleDataReceived to drop the message.

When you add *ENCRYPT, this causes a major deadlock. *ENCRYPT will queue most messages until it gets the encryption key, which can often include the first message (so UNICAST3 won't see it yet). Then when an important message such as JOIN_RSP comes through, UNICAST3 drops it. Since UNICAST3 never lets any messages through in this case, the encryption key will never get set so that *ENCRYPT can pass the first message up and free the deadlock.