Don't send receiver reports for SSRC during simulcast probe

[anders-avos]

libwebrtc will stop sending SDES headers after the first receiver report on an SSRC^1. This breaks simulcast probing if the receiver report is sent before probing is completed. This change delays receiver reports for probing SSRCs until the first non-probe packet is received.

I didn't find any easy way to get this information into the interceptor, so I opted for using attributes. Let me know if you prefer some other means.