ortp: detect ssrc changes immediately
ortp: detect ssrc changes immediately

libortp detects an ssrc jump using a packet count thresold. This
threshold is set to 50 by default. This means that libortp has
to see 50 rtp packets with the new SSRC before it accepts the
new stream. In our application we want to detect the changed
SSRC (Handover) as fast as possible. So we should change the
threshold to zero.

- use rtp_session_set_ssrc_changed_threshold() to set a thresold
  of 0 packets when osmo_rtp_socket_create() is called.

Change-Id: Id88c2c44d29a409affc2e47fdb932e1187020593
Related: OS#3299

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis