dnsdist: Keep watching idle DoH backend connections

dnsdist: Keep watching idle DoH backend connections

So we can quickly detect a connection closed by the remote host. The
`isTCPSocketUsable()` function is unfortunately not enough if the
remote end properly sent a GO AWAY frame before closing the connection,
as the socket then becomes readable and usable, instead of closed. We
need to actually read that message to know that the remote end closed
the connection.