Draft: assorted changes for DTLS-SRTP support in Farstream

These are assorted changes that were necessary to get DTLS-SRTP for (1:1) RTP conferences going in Farstream, plus changes necessary for successful negotiation of RTP sessions using Gajim's Jingle implementation.

NOTE: this is not yet ready for merging in the slightest. I'm just putting it out here to give the idea of the changes required, and maybe get some feedback on how to do things properly.