dnsdist: Prevent a cleanup race between the DOHUnit and the request pool

dnsdist: Prevent a cleanup race between the DOHUnit and the request pool

- clean up the pointer in pool memory when releasing a DOHUnit so that we
  don't try to access it later when the memory pool is destroyed ;
- clean up the 'self' pointer when the memory pool is destroyed so we
  don't try to access it when the DOHUnit is released.