[9.18] fix: dev: Unify the int32_t vs int_fast32_t when working with atomic types

There's a mismatch between the atomic and non-atomic types that could
potentialy lead to a rwlock deadlock (after two billion 2^32) writes.
Use int_fast32_t when loading the atomic_int_fast32_t types in the
isc_rwlock unit.

Closes #5280

Merge branch '5280-match-the-types-in-isc_rwlock-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10390