* Create an initialise a ZRTP context on a channel when a ZRTP exchange was already performed on an other one
* @param[in] stream_sessions A link to the stream sessions structures, used to get rtp session to add transport modifier and needed to set SRTP sessions when keys are ready
* @param[in] activeContext The MSZRTP context of the already active session, used to pass to lib bzrtp its own context which shall remain unique.
* @param[in] params ZID cache filename and peer sip uri
* @return a pointer to the opaque context structure needed by MSZRTP