Some problems when we release the device:

    - Calling the destructors is wrong, we should set the pointers to NULL.
      These are shared pointers, this is C++.
    - We have to stop rx by doing a STREAM_MODE_STOP_CONTINUOUS.
    - The end-of-burst for tx has been simplified.

This problem when we acquire the device:

    - Calling get_rx_stream() many times seems to be problematic.

We now should have a better behavior with UHD 4.2 and N310. (Hopefully with
others too.) No crash seen so far.