Pointers to listen_socket_t are stored in dht_tracker, so they
cannot be copied or moved once they are added to m_listen_sockets.
listen_socket_t should really be made non-copyable and non-moveable, but the
socket creation path needs to be refactored before that can be done.
Moved the logic to determine which sockets to keep to a separate function and
add unit tests for it.
Store the port which was originally specified for a listen socket so that we
can match sockets reliably even with port retries.