dnsdist: Properly handle reconnection failure for backend UDP sockets

dnsdist: Properly handle reconnection failure for backend UDP sockets

We try to reconnect our UDP sockets toward backends on some kind of
network errors that indicate a topology change, but we need to be
careful to handle the case where we actually fail to reconnect, as
we end up with no remaining sockets to use.
This commit properly deals with this case by pausing the thread handling
UDP responses from the backend, instead of having it enter a busy loop,
and by attempting to reconnect if we get a `bad file number` error when
trying to send a UDP datagram to the backend.