Root Zone local copy, similar to RFC 7706

Enable a resolver to download, validate, cache and update the entire root zone, obviating the need for queries to the root zone.
ICANN is sponsoring ISC to add this feature.

• Refactor zone logging functions #269 (closed)
• constify dns_rdata_tostruct #341 (closed)
• Convert verifyzone() to a libdns function #266 (closed)
• Unify keyfile-to-configuration conversions in system tests #284 (closed)
• Implement mirror zones #33 (closed)