Unify the int32_t vs int_fast32_t when working with atomic types

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.