Add "Delay-A: ms", "Delay-AAAA: ms" and "Delay-HTTPS: ms" to the test
dnsd config and implement delayed response handling.
Add test_21_09 and test_21_10 to check that delayed responses connect
using the undelayed address family.
Closes#21299
Add `resolv-threaded` to curlinfo to detect use of the threaded resolver
correctly even with c-ares linked to https-rr.
Run test_21_05 exactly when threaded resolver is built.
Closes#21287
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
