rtp: Randomize the initial timestamp for RFC compliance

Pick a random timestamp when creating the session.

RFC 3550:
The initial value of the timestamp SHOULD be random, as for the
sequence number.

Fixes: SYS#403
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index 89a69ee..fedc977 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -339,6 +339,7 @@
 
 	/* initialize according to the RFC */
 	rtp_session_set_seq_number(rs->sess, random());
+	rs->tx_timestamp = random();
 	
 
 	return rs;