tell device to send RTP to asterisk on remote bridge end

This fix quite a few SCCP related bugs, but doesn't work well with a
vanilla asterisk 11 since there's a race condition in the core that is
exacerbated by this fix.
don't implement ast_channel_tech::send_digit_begin

Does not work well with directmedia and feature codes.

Ticket #4962.
check if subchan is active instead of using on_hold member

Also do the check in update_peer a bit earlier, so that we don't send a
start/stop media transmission for an inactive subchan.
stop media only for active subchannels

Otherwise, when a call that is on hold is hanguped from the remote side,
this break the other communication on certain phone models.
remove redundant call to sccp_subchannel_get_rtp_local_address