dnsdist: Increment the DOHUnit ref count when it's set in the IDState

dnsdist: Increment the DOHUnit ref count when it's set in the IDState

We need to increment the reference counter even before sending the
query to the backend, as soon as we copy a reference into the IDState.
Because:
- that makes sense anyway, we are storing a new copy ;
- otherwise, in the unlikely event where we reuse the IDState before
  the query has been sent to the backend we might free the DOHUnit
  before the reference counter has been incremented and cause a
  double-free.