rsl: Output RTP stats before closing the socket.
rsl: Output RTP stats before closing the socket.

It's useful to know RTP statistics (number of packets lost, jitter, etc)
when looking at voice call quality issues. Right now this information is
not avialable anywhere and this looks like the best place to start.

Change-Id: Ife9f27c43157b4a1bf71aba41cd7b0f5f41ac99f

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts