ASCI: Support conference briding with 1..n connections

An RTP bridge may have 1 or many endpoints. Each endpoint may receive,
send or echo audio back. Because transcoding is not supported, it must
be assumed that only connection receives RTP in a conference with more
than 2 connections.

A connection that receives RTP into the endpont will have "send" pointer
set to (one of) the other connection that shall send out RTP.
All connection that send RTP have "send_next" pointer set to the next
or first connection that sends RTP. They are linked in a ring.

Regular bridge with two connections: Each connection forwards audio
towards the other connection and only towards the other connection.
The "send_next" pointers are set to NULL or pointing to the other
endpoint.

connection A                            connection B
("sendrecv")                            ("sendrecv")
conn->send --------------------------->
 <------------------------------------- conn->send
conn->send_next ---------------------->
 <------------------------------------- conn->send_next

Conference bridge with three connections: Each connection forwards
audio to itself and also towards all other connection.

connection A        connection B        connection C
("confecho")        ("confecho")        ("confecho");
conn->send --\      conn->send --\      conn->send --\
 <-----------/       <-----------/       <-----------/
conn->send_next -->
                    conn->send_next -->
 <------------------------------------- conn->send_next

The same conference bridge but without echo: Each connection forwards
auto to all other connection but not to itself.

connection A        connection B        connection C
("confecho")        ("confecho")        ("confecho");
conn->send -------> conn->send ------->
 <------------------------------------- conn->send
conn->send_next -->
                    conn->send_next -->
 <------------------------------------- conn->send_next

Change-Id: Ic99a55ab5a3a6170e940403fadd52697e99f2f3a
---
M include/osmocom/mgcp/mgcp_trunk.h
M src/libosmo-mgcp/mgcp_endp.c
M src/libosmo-mgcp/mgcp_network.c
M src/libosmo-mgcp/mgcp_protocol.c
M src/libosmo-mgcp/mgcp_trunk.c
5 files changed, 190 insertions(+), 60 deletions(-)