ref(JitsiConference): move sendTones impl to TPC (#983)
Moves sendTones implementation to TraceablePeerConnection. It will be
responsible for creating/storing a DTMFSender. JitsiConference will
use the currently active TraceablePeerConnection to send the tones.
This fixes a bug where DTMF tones can not be sent after
JingleSessionPC/TraceablePeerConnection instance changes.
The JitsiDTMFManager instance was referencing invalid PeerConnection.