async-threaded resolver: use ref counter

async-threaded resolver: use ref counter

Allocate the data shared between a transfer and an aync resolver thread
separately and use a reference counter to determine its release.

Change `Curl_thread_destroy()` to clear the thread handle, so that the
thread is considered "gone" and we do not try to join (and fail to)
afterwards.

Retake of the revert in fb15a986c0d947ae6b9dd6

Closes #16916