Mesh LPN RX race condition

Suppose node L is in a friendship with node F. Consider the following: 

1. Some other node C sends a message to L. 

2. F hears the message, and saves the message into L's friend queue.

3. L also happens to be scanning (e.g. L is currently in a Friend Poll receive window), and hears the message itself.

4. F sends the recorded message to L as the Friend Poll reply.

5. However, when L receives the recorded message from F, it will discard it on the Network layer due to message cache hit (it has already seen the message itself in step 2). This happens in the 3rd if statement in `deobfuscated_header_is_valid`.

6. Therefore, the LPN stack on L never receives L's Friend Poll reply. It thus never updates fsn, and will keep requesting the same message from L. 

7. After 5 Friend Polls, the LPN stack on L will terminate the friendship. 

I think in this case the LPN stack should still somehow increase its fsn despite of the message cache hit. 

To reproduce this, I had C continuously sending messages to L (3 messages every second). It usually reproduces within the first 20 seconds or so. 

Thanks!