dns: stop polling for updates; use UpdateState API

[dfawley]

This cannot be merged until the pickfirst and round_robin balancers are updated to return an error from UpdateState when zero addresses are provided, as we currently rely upon the polling behavior of DNS to refresh on error.

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This change is 

[dfawley]

This PR is ready for review now that #3186 is merged.

[menghanl]

Reviewed 1 of 2 files at r1.
Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @dfawley and @menghanl)

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internal/resolver/dns/dns_resolver.go, line 297 at r1 (raw file):

		Addresses: append(d.lookupHost(), srv...),
	}
	// Support fallback to non-balancer address.

Move this comment above append.
And make it clear what it means? Addresses contain fallback non-balancer addresses from host and balancer addresses from SRV

Or just delete it.

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internal/resolver/dns/dns_resolver_test.go, line 753 at r1 (raw file):

		}
		sc := scFromState(state)
		if !reflect.DeepEqual(a.scWant, sc) {

This is comparing strings, ==

[dfawley]

Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @menghanl)

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internal/resolver/dns/dns_resolver.go, line 297 at r1 (raw file):

Addresses contain fallback non-balancer addresses from host and balancer addresses from SRV

Deleted, since we will be getting rid of the balancer addresses in the address list soon.

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internal/resolver/dns/dns_resolver_test.go, line 753 at r1 (raw file):

Previously, menghanl (Menghan Li) wrote…

This is comparing strings, ==

Done.

[menghanl]

Reviewed 1 of 1 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved