Enable requesting TCP connections to be closed

Enable requesting TCP connections to be closed

In response to client queries, AsyncDnsServer users can currently only
make the server either send a reply or silently ignore the query.  In
the case of TCP queries, neither of these actions causes the client's
connection to be closed - the onus of doing that is on the client.
However, in some cases the server may be required to close the
connection on its own, so AsyncDnsServer users need to have some way of
requesting such an action.

Add a new ResponseAction subclass, ResponseDropAndCloseConnection, which
enables AsyncDnsServer users to conveniently request TCP connections to
be closed.  Instead of returning the response to send,
ResponseDropAndCloseConnection raises a custom exception that
AsyncDnsServer._handle_tcp() handles accordingly.