dns: use handles to prevent use-after-free problems between dns queries and callback targets

Description: ActiveDnsQueries have independent lifetimes from their callback targets (strict/logical dns cluster, dns cache). Right now there is an implicit contract where the callback target sets their query pointer to null once the query is resolved, and the query only fires callbacks if it has not been cancelled. However, this implicit contract can create problems when it's not adhered to (e.g #10006).

An enhancement would be to use handles with RAII constraints similar to how the init manager does with watchers and targets.