Requesting too many new keyframes

When running the owr daemon with some additional printfs in the code where keyframes are requested in gstrtpsession.c, one can see that in some situations 10 or so keyframes are requested per second... and then many keyframes arrive in a row.

We should probably throttle this a bit somewhere. If a keyframe got lost because of network saturation, it's not going to get better if we saturate the network even more ;)

Start in gstrtpsession.c at the code paths where keyframes are requested, using the reported printfs to reproduce the repeated requests. Determine a throttling rule that prevents bursts during network saturation, then verify that keyframe requests no longer occur around ten times per second in the affected situations.

Written by the indexing model from the issue text.