[AVTCORE] Key diversification with DTLS-SRTP with Shim

[Eric Rescorla <[e-mail]>]

Section 6.3.2 of
http://tools.ietf.org/id/draft-westerlund-avtcore-transport-multiplexing-04.txt
states:

 Instead we propose that an DTLS-SRTP key-derivation change is
   introduced.  By including the Session ID value in the derivation of
   the keying material a single DTLS-SRTP key-management operation could
   apply keys and parameters for all the RTP sessions in the same
   transport flow.  Thus the keying cost is significantly reduced,
   especially in regards to network communication and delay impact and
   vunerability to packet loss.

This seems totally reasonable.

DTLS-SRTP uses RFC 5705 TLS Exporters. These take an optional "context" value
which is currently not used. I propose we set the "context" to be the shim ID
(represented as a 16-bit big-endian unsigned integer).

-Ekr