Replace lock keyfile hashmap with lock pool

Replace lock keyfile hashmap with lock pool

Kasp used a lock per zone origin in order to prevent concurrent access
to keyfiles. This lead to substantial memory consumption in the case of
authoritative servers with many small zones, as lots of locks need to be
allocated.

Since the number of keyfile locks taken cannot exceed the number of
helper threads, it makes more sense to use a lock pool of fixed size
keyed by the hash of the origin name, leading to memory savings.