dnsdist: Reset origFD asap to keep the outstanding count correct

dnsdist: Reset origFD asap to keep the outstanding count correct

Previously the health check thread waited until we had finished
with the IDState to set `origFD` to -1, but:
* for the UDP client thread, the only difference it makes is that
`outstanding` will not be incremented if `origFD` is not -1,
which is not what we want since we are going to decrement it
* for the UDP responder thread, it actually increases the
likelihood of decrementing `outstanding` twice, once in the
responder threader and once in the health check thread.

This was especially likely to be an issue because the health check
thread used to call `gettime()` and to acquire a mutex before
setting `origFD` to -1.