allow tcpdns sockets to self-reference while connected

allow tcpdns sockets to self-reference while connected

A TCPDNS socket creates a handle for each complete DNS message.

Previously, when all the handles were disconnected, the socket
would be closed, but the wrapped TCP socket might still have
more to read.

Now, when a connection is established, the TCPDNS socket creates
a reference to itself by attaching itself to sock->self. This
reference isn't cleared until the connection is closed via
EOF, timeout, or server shutdown. This allows the socket to remain
open even when there are no active handles for it.

(cherry picked from commit cd79b4953800daf87b6bfcea0b6f3ce3bec29988)