Use a thread queue and pool for asnyc threaded DNS resolves.
Add pytest test_21_* for verification.
Add `CURLMOPT_RESOLVE_THREADS_MAX` to allow applications to
resize the thread pool used.
Add `CURLMOPT_QUICK_EXIT` to allow applications to skip thread
joins when cleaning up a multi handle. Multi handles in
`curl_easy_perform()` inherit this from `CURLOPT_QUICK_EXIT`.
Add several debug environment variables for testing.
Closes#20936
