Address secdir early comments

[ShivanKaul]

• Cleanup terminology section; remove APEX definition and define Verifying Record, fix https://github.com/ietf-wg-dnsop/draft-ietf-dnsop-domain-verification-techniques/issues/45
• RECOMMEND the underscore prefix label explicitly for the verifying record
• Put requirements on the Random Token instead of prescribing how to generate it
• Cleanup metadata expiry advice
• Move security-relevant text about clear records to Security Considerations section
• Move AVOID-FRAGMENTATION ref to info
• Minor cleanups

Fix https://github.com/ietf-wg-dnsop/draft-ietf-dnsop-domain-verification-techniques/issues/63