lib/dns/zone.c refactoring

While reviewing a MR that touches lib/dns/zone.c, I have notice several things:

• zone_postload() needs to be refactored, the functions spans from line 4696 to to line 5290 (e.g. 400 lines)
• zone_rekey() also need to be refactored as it spans from line 19317 to line 19734
• keyfetch_done() spans from line 9993 to line 10647 (600 lines)
• zone_nsec3chain() ~900 lines
• The locked parameter in zone_locked() should be removed in favor of just locking the zone prior to the call
• There's probably a missing lock in dns_zone_catz_enable_db()
• There's probably a missing lock around dns_zone_rpz_disable_db() and dns_zone_catz_disable_db() call
• There's lot of unlocked access to dns_zone_t members in zone_nsec3chain() and zone_sign() at the top of the function
• There's unlocked access to dns_zone_t members in zone_maintenance() (zone->view, zone->type, zone->masters, ...)
• There's unlocked access to zone->maxrecords in dns_zone_getmaxrecords() and dns_zone_setmaxrecords()
• There's unlocked access to zone->origin, zone->flags, zone->dblock, and other members of dns_zone_t in zone_notify()
• zone_rekey() contains unlocked access to zone->mctx, zone->origin, zone->rdclass and other members

The list here is neither complete or necessarily correct. Some of the struct members might be read-only (e.g. protected by reference counting), but the struct comment is saying "/* Locked */", so if they are truly read-only, perhaps casting them to const would be a right thing to do. Also I mostly checked only static functions and looked only at dns_zone_t * accesses.