Merge branch 'bulk_updates' into 'master'

Bulk updates

Pending:
 - ~~renaming (update vs. request vs. query ...)~~
 - ~~proper logging (waiting for !250)~~
 - ~~`rrset_is_nsec3_rel()` and `mm_realloc` copypasta~~
 - partial zone adjustment - next MR

Pros:
 - faster for lots of small updates
 - improved terrible empty node cleanup (now takes time proportional to size of update, not size of the zone)
 - spinlocks instead of mutexes for ddns q access (faster for this case + less memory :fisted_hand_sign: )

Cons:
 - if one transfer fails (no matter the reason, except for acl issues), they all fail
 - might actually be slower for small singular updates, but oh well

Not sure if good or bad:
 - most of logging is lost (only logs errors in detail, i.e. who sent the faulty update)

See merge request !251