Fix memory leaks and use-after-free detected by ASan

[jfclere]

Summary:

• a2md: fix allocator leak by setting pool ownership and destroying pool on exit
• md_crypt: fix EVP_PKEY leak by moving cleanup registration into make_pkey()
• md_crypt: fix ASN1_TIME leak in self-signed cert creation
• md_crypt: fix AUTHORITY_KEYID leak in md_cert_get_ari_cert_id()
• md_acme: fix use-after-free accessing req after on_response already freed it
• md_json: fix stack-use-after-return in md_json_itera() and md_json_iterkey()
• mod_ssl: fix cipher list leak in ssl_hook_Access_classic() on early return for secondary connections
  Note that pyhttp testsuite triggers those.
  coauthored by Claude Opus 4.6