Transceiver52M: Introduce usage counting for DriveLoop and RadioInterface.

The reason is to allow them to be started from any TRX. I.e. they are started
when the first TRX starts and stopped when the last TRX stops.
Transceiver52M: Start and stop all threads on POWERON/POWEROFF.

This change reveals some issue with the UHD start/stop code. Specifically,
we get a failed assert in RadioInterface::pullBuffer() (num_rd == OUTCHUNK
with num_rd=0) with the POWERON/POWEROFF/POWERON sequence.
Transceiver52M: HACK: Exit transceiver on POWEROFF command.

This hack allows OsmoBTS to correctly restart itself with different parameters.
We assume that transceiver is run as a service and will be restarted on exit.