Changes

do not FAIL on CRCX in sendrecv mode

Currently, a CRCX in sendrecv mode results in:

  DLMGCP ERROR endpoint:rtpbridge/2@mgw CI:7F4C8EDD CRCX: selected connection mode type requires an opposite end! (mgcp_protocol.c:1090)

But it is not actually practical, nor logical to require another conn
for the 'sendrecv' ConnectionMode.

Impractical: If I want to create two conns on an endpoint that both are
in 'sendrecv' mode, I have to send two CRCX, one for each conn. At the
time of the first CRCX, there cannot be any other conn, so I am
currently forced to send a different ConnectionMode in the CRCX,
followed by another MDCX later, just to change the first conn to
sendrecv.

Illogical: In a situation where two conns are currently in sendrecv
mode, if I now DLCX one of them, I can legally reach a state with a
single conn in sendrecv mode, just as currently forbidden for CRCX.

In general, MGCP is not forcing any particular number of connections.

Simply start forwarding RTP as soon as there is a remote conn, and not
require another explicit MDCX.

Related: SYS#6974 SYS#6907
Related: osmo-ttcn3-hacks I00fd854f058f7f53e2f579e8481ca2b9253f08e3
Change-Id: Ic089485543c5c97a35c7ae24fe0f622bf57d1976