In #475, import low-level API (ReadMsgUDP) to determine
destination interface for packets received by UDPConn listen
at any(unspecified) address, to fix msg received by incorrect
candidate that shared same ufrags. But the api has compatibility
issues, also not reliable in some special network cases like
AWS/ECS.
So this pr revert that change, and make UDPMuxDefault not
accept Conn listen at unspecified address. Also provide a
NewMultiUDPMuxFromPort helper function to create a MultiUDPMux
to listen at all addresses.
For ice gather, it will use UDPMux's listen address to generate
canidates instead of create it from interfaces.
When we have multiple host candidates and mux to a
single port, if these candidates share a same conn
(either tcp or udp), they might read other's msg
cause ice connection cost long time or failed.
These wrappers allow a caller to provide UDPMux and TCPMux instances
to the ICE agent that represent multiple open ports. This can be
desirable in what would otherwise be single-port deployments, as it
increases the chance that one of the fixed ports will not be blocked
by a users firewall.
