Add an extra reference to session for VideoRoom participants

[lminiero]

This is a PR to address a different crash caused by a race condition, as reported in #3154.

[tmatth]

@lminiero does this need a backport?

[lminiero]

I'm not sure: this patch, and #3167 before it, was quite focused on how the multistream version of the VideoRoom plugin works. I'm not aware of similar issues in 0.x. That said, this is generic enough that it might make sense.