ASCI: Support conference briding with 1..n connections

For each RTP packet that is received from a connection, the mode is
checked whether receiving is allowed or not. If not it is discarded.

In case of "confecho" mode, the RTP is also sent by the receiving
connection.

Then a loop is used to send RTP to all sending endpoints except the one
that received the packet.

Because we have a loop that allows to have 1..n connections, we have no
maximum number of allowed connections anymore.

Change-Id: Ic99a55ab5a3a6170e940403fadd52697e99f2f3a
Related: OS#4853
---
M src/libosmo-mgcp/mgcp_conn.c
M src/libosmo-mgcp/mgcp_endp.c
M src/libosmo-mgcp/mgcp_network.c
M src/libosmo-mgcp/mgcp_protocol.c
4 files changed, 52 insertions(+), 30 deletions(-)