dnsdist: Reset origFD asap to keep the outstanding count correct

[rgacogne]

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.

This might help with #4344.