webrtcbin, rtpbin: check before setting properties on jitterbuffer

In rtpbin we already systematically check for all property names except latency, correct that.

In webrtcbin we need to check before trying to use the do-retransmission property.

This is useful for the case where an element like identity gets passed to rtpbin's request-jitterbuffer property, when the application wants to use webrtcbin in an SFU situation, with no reordering and no added latency