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 improvement.

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

[ming]

In 5792:

Closed #2113: Implement conference signal level adjustment for a specific connection