dns: do not call NewServiceConfig when lookups are disabled

[dfawley]

Also:

• Fully ignore NewServiceConfig calls in resolver_conn_wrapper
• Fix dns_resolver_test to actually test service configs, and a handful of cleanups

Fixes #3199

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This change is 

[menghanl]

Reviewed 2 of 3 files at r1.
Reviewable status: 2 of 3 files reviewed, 1 unresolved discussion (waiting on @dfawley and @menghanl)

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internal/resolver/dns/dns_resolver_test.go, line 614 at r1 (raw file):

	var r []string
	for i := 0; i < len(b); i += txtBytesLimit {

Copy copy the comment with the code?

[menghanl]

Reviewed 1 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @dfawley)

[dfawley]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @menghanl)

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internal/resolver/dns/dns_resolver_test.go, line 614 at r1 (raw file):

Previously, menghanl (Menghan Li) wrote…

Copy copy the comment with the code?

Done.

[menghanl]

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