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:

• locally (Linux)
• 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)