Handle concurrent verifiers better if zone is being fed over stdin

[k0ekk0ek]

Users are seeing better performance/concurrency when verifiers use (A|I)XFRs for zone validation. This is due to the fact that the verify process keeps writing to a single verifier until it's done (see verify_handle_feed in verify.c:220). In the future we should consider writing smaller chunks and alternating between verifiers to improve throughput and thereby loading times.