6.4.0

CHANGES:

• dns-client: demote log level of response to debug (#317 @hannesm)
• dns-client: use DNS-over-TLS for uncensoreddns.org only (#320 @hannesm)
• API: dns-client: connect returns the protocol (UDP/TCP), allowing mixed UDP
  and TCP namerservers being used (#322 @hannesm)
• dns-client-mirage: allow hostname in authenticator, improve error message and
  documentation (#319 #322 @hannesm)
• dns-client-mirage: support UDP nameservers as "udp:" in
  nameserver_of_string (#322 @reynir @hannesm)
• API: dns-client, dns-stub, dns-resolver: ?size is now ?cache_size (#322
  @hannesm, suggested by @reynir)