remove isc_task and related APIs

The isc_loop, isc_async and isc_job APIs can now do everything isc_task and isc_event used to, and in fact the latter is just a shim around the former. Let's convert everything that still uses isc_task to use loop instead:

• zone.c (zonemgr_getio() and the dns_io API, key refresh queries, synchronizing inline signing zones, nsec3param updates, xfrin quota, etc)
• isc_ratelimiter
• libdns modules: dns_request, dns_validator, dns_adb
• dnssec-signzone
• UPDATE processing
• plugin, dyndb, and dns_client callbacks
• TAT queries
• dns_resolver
• dns_catz and rndc addzone/delzone
• final isc_task API cleanup