RRSet refactoring - remove RRSIGs, custom memory allocator

- `rrset->rrsigs` removed + code removal where applicable
- mem_ctx_t added to RRSet API, only used in pkt.c now

## State of the review

Up to `nsec_proofs.c`

- Compression hint is now stored in the rrsig_info structure and reused.

- Most changes were in ddns.c: Empty checks used rr_count == 0

- RR insertion function now returns zone RRSet.
- Removed reduntant zone contents / node API functions
- Cleanup.

- Fixed mismatched variables, do not use call directly in for loops.

(processing locks RCU per-response, but zone content must not change
 during the iterations)

fixes #215

- previously static function 'put_rr' in internet.c has been made public, it handles additions of RRSIGs as well now, by adding the to a list that is traversed after each section has been solved.