The srtp_update and srtp_update_stream functions are provided
as a way to rekey or update the policy of an existing stream while
preserving the ROC as required by
https://tools.ietf.org/html/rfc3711#section-3.3.1 .

The current implementation only preserves the ROC. It does not
preserve the replay database for RTP or RTCP, that is a possible
improvement.