Calling libusb_cancel_transfer only starts the cancellation of a
transfer. The process is not complete until the transfer callback
has been called with status LIBUSB_TRANSFER_CANCELLED.
If hackrf_start_rx() is called soon after hackrf_stop_rx(),
prepare_transfers() may be called before the previous cancellations
are completed, resulting in a LIBUSB_ERROR_BUSY when a transfer is
reused with libusb_submit_transfer().
To prevent this happening, we keep track of which transfers have
finished (either by completion, or cancellation), and make
cancel_transfers() wait until all transfers are finished.
This is implemented using a pthread condition variable which is
signalled from the transfer thread.
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