src: lib: stream: Improve RTSP latency

[joaoantoniocardoso]

The is-live=true in shmsrc was adding ~1 frame of latency, while the sync=true in shmsink was adding more ~2.

tested:

• [x] locally (Linux)
• [x] BlueOS @ Raspberry Pi 4

How to test:

1. Create a QrTimestamp 320x320@30fps RTSP stream as below:

2. To receive it, run the following line:

   clear; GST_DEBUG="*:3,*qrtimestampsink*:5" gst-launch-1.0 -v \
   rtspsrc location=rtsp://192.168.100.2:8554/test is-live=true latency=0 \
   ! application/x-rtp,payload=96 \
   ! rtph264depay \
   ! h264parse \
   ! avdec_h264 discard-corrupted-frames=true \
   ! videoconvert \
   ! qrtimestampsink

Before this patch (master), the expected latency is ~100 ms. After this patch it is ~34 ms (which is the same as the UDP)