Improve lease extended info

Followup of the #2587 (closed) ticket:

• extend sanity-checks with an extended-info entry doing:
  • ignore (do nothing: use with care as it can lead to undetected inconsistencies)
  • fix (default: will fix common inconsistencies, this includes upgrade to new format)
  • strict (fix + extra checks which makes sense only with Bulk Lease Query. Could be the default when BLQ is loaded)
  • pedantic (all checks which can be done even useless for BLQ)
• log sanity check action
• update doc

Likely incomplete list of checks with the fix and stricter variants:

• user context is not null but not a map: make it null
• "ISC" entry is not null but not a map: remove it
• old format: move to new format and continue checks
• both old and new formats (v6): remove old format and likely obsolete entry
• extended-info entry not null but has a wrong type: remove it (chain)
• v6 empty list: remove it (mainly because it makes current LQ code to refuse it, chain)
• (sub)option entry not null but not a string: remove extended info (chain)
• can't decode (sub)option hex string: remove extended info (chain)
• (extra, v6) no link address: remove extended info (chain)
• (extra, v6) link address is not a string: remove extended info (chain)
• (extra, v6) link address string is not an IPv6 address: remove extended info (chain)
• (extra) relay-id is not null and not a string: remove extended info (chain)
• (extra) relay-id hex string can't be decoded: remove extended info (chain)
• (extra) remote-id is not null and not a string: remove extended info (chain)
• (extra) remote-id hex string can't be decoded: remove extended info (chain)
• (pedantic, v6) missing fields: hop and peer address: remove extended info (chain)
• (pedantic) extra fields: remove extra fields other than "comment"

Chain means if the (map) parent of the removed entry becomes empty remove it recursively.

Two things out of the scope of this ticket:

• extension to not memfile backend
• write the updated lease to the file (this includes a possible support by LFC)