Rest of the code rewritten to use memcontext for RRSet API

src/knot/ctl/knotc_main.c

src/knot/dnssec/nsec-chain.c

src/knot/dnssec/nsec3-chain.c

src/knot/zone/zone-load.c

src/utils/common/exec.c

src/utils/dig/dig_exec.c

src/utils/nsupdate/nsupdate_exec.c