Add more durations and plotting to benchmark.

This series adds a push, receive and decode times to the latency benchmark.

The benchmark is now evaluated from the following timestamps:

• server_render_begin_time: Server time at compositor begin.
• server_push_time: Server time when pushing the frame to the GStreamer appsrc. The timestamp was added to the DownMessage.
• client_receive_time: Time at depayloading the rtp buffer on the client
• client_decode_time: We already measured this, for consistency i converted it to int64_t from timespec.
• client_render_begin_time: New timestamp which takes into account retrieving the GL image from the gstreamer pipeline.

The resulting durations are now logged as json. The evaluation script was updated and can now plot the data.

Example plot