Create a multicast-only UDP server listening for multicast packets towards multi_addr.
The server will only receive packets towards multi_addr,
no unicast traffic will be received.
Multicast only servers created via UdpServer::multicast are shared.
There can be multiple such server listening on the same address at the same time.
The server will automatically join the IP multicast group at multi_addr,
using interface interface_addr (defaults to a random interface chosen by the OS).
Only the IP part of interface_addr matters, the port is irrelevant.
Due to OS limitation, currently every multicast-only server
can listen for only one multicast IP.
Calling join_multicast_group on server created by UdpServer::multicast
with different address simply does not work.
Users can join the same IP with a different interface using join_multicast_group, though.
Currently this function is IPv4 only.
There is no simple way to implement multicast-only IPv6 socket on Linux/MacOS.