Initial implementation of XFR bootstrap.

libknot/nameserver/name-server.c

libknot/rdata.c

libknot/updates/ddns.c

libknot/updates/xfr-in.c

libknot/updates/xfr-in.h

libknot/util/debug.c

libknot/zone/zone.c

libknot/zone/zone.h

libknot/zone/zonedb.c

src/knot/server/xfr-handler.c

src/knot/server/zones.c

src/knot/zone/zone-load.c