Implement conference signal level adjustment for a specific connection

[ming]

In the doc of the API pjmedia_conf_connect_port(pjmedia_conf *conf, unsigned src_slot, unsigned sink_slot, int level):

@param level         This argument is reserved for future improvements
                     where it is possible to adjust the level of signal
                     transmitted in a specific connection. For now,
                     this argument MUST be zero.

This ticket will implement that.

Thanks to Michael Scheiffler for the original patch and Thibault for improvements and fix.