Handle fNS XmitDataAns as a TxAck

[conny-andersson]

I made this change to fix issue #555 but I can see now that you did some additional changes to the code and removed the async XmitDataReq.

If you have time to look at this solution, it treats the XmitDataAns as an TxAck and fires that flow. If this is not done, frame-counters are never updated and subsequent downlinks will not be accepted by the end node. I made this async so that the downlink session is saved before TxAck handling is started, similar to how it is handled in fNS.

[brocaar]

Thanks @conny-andersson-netmore 👍 I took your code as an example to implement the above commit. Instead of syncing the Go routines using channels, I added this as an extra step to the flow.

If the XmitDataReq fails, this point is never reached and the TxAck is never triggered. If the XmitDataAns succeeds, then the device session is saved before the TxAck is triggered.