module-rtp: add timer for ck requests

Scale RTP timestamps against the clock, allow some jitter.
Make method to query current RTP timestamps.

src/modules/module-rtp/stream.h

@@ -42,6 +42,8 @@ void rtp_stream_destroy(struct rtp_stream *s);
 
 int rtp_stream_receive_packet(struct rtp_stream *s, uint8_t *buffer, size_t len);
 
+uint64_t rtp_stream_get_time(struct rtp_stream *s, uint64_t *rate);
+
 
 #ifdef __cplusplus
 }