processing: remove UCW list from the API, refactor usage

src/knot/modules/dnsproxy.c

src/knot/nameserver/update.c

src/knot/nameserver/update.h

src/knot/server/tcp-handler.c

src/knot/server/udp-handler.c

src/knot/zone/events/handlers.c

src/knot/zone/events/replan.c

src/knot/zone/zone.c

src/libknot/internal/lists.c

src/libknot/internal/lists.h

src/libknot/processing/layer.h

src/libknot/processing/overlay.c

src/libknot/processing/overlay.h

src/libknot/processing/requestor.c

src/libknot/processing/requestor.h

tests/overlay.c