Connections routed to a selector (route target or final) enter through
the ConnectionHandler path (NewConnection/NewPacketConnection), which
never goes through Selector.DialContext/ListenPacket, so they were
never registered in the interrupt group. As a result, SelectOutbound
interrupted nothing and existing connections kept flowing over the
previously selected outbound.
Register the inbound connection in the interrupt group as an external
connection on both handler branches, so switching the selected outbound
interrupts routed connections when interrupt_exist_connections is
enabled, while leaving them untouched by default.
Fixes#4281.
zhxu
07460d1a7a