Use thread_local EVP_MD in isc_iterated_hash()
Cherry-pick small fixup commit from 9.18 branch needed for thread-safety. This fixup commit is not needed for 9.19+ because of reworked application setup, but it decouples isc_iterated_hash and isc_md units and keeps all the branches in sync.
Closes #3795 (closed)
Edited by Ondřej Surý