chg: dev: Optimize memory layout of core structs

Reduce memory footprint by:
- Reordering struct fields to minimize padding.
- Using exact-sized atomic types instead of `*_least`/`*_fast` variants
- Downsizing integer fields where possible

Affected structs:
- dns_name_t
- dns_slabheader_t
- dns_rdata_t
- qpcnode_t
- qpznode_t

Closes #5022

Merge branch '5022-reduce-metadata-overhead-by-struct-packing' into 'main'

See merge request isc-projects/bind9!9721