refactor dns_resolver to use loop callbacks

Evan Hunt requested to merge 3797-part7-resolver-refactoring into main

callback events from dns_resolver_createfetch() are now posted using isc_async_run.

other modules which called the resolver and maintained task/taskmgr objects for this purpose have been cleaned up.

See #3797 (closed)

Edited by Michał Kępień

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