v2.4.0: Newhope (#165)

* change DEFAULT_SYS from dnsdb to dnsdb2; adapt rest of system to cope

* add support to probe what pDNS systems the endpoint supports

* normalize DNSDB APIv2; rename encap_bare to encap_cof; incorporate encap into pdns_system; remove all encap inequality tests

Co-authored-by: Paul Vixie <[email protected]>
Co-authored-by: David Waitzman <[email protected]>