Backport of consul: do not re-register already registered services into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14917 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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This PR updates Nomad's Consul service client to do map comparisons
using maps.Equal instead of reflect.DeepEqual. The bug fix is in how
DeepEqual treats nil slices different from empty slices, when actually
they should be treated the same.

Also, we need to avoid comparing TaggedAddress on agent registrations if
none are set on the Nomad side - because Consul will helpfully set that field
for us on its side.

And some drive-by cleanup map[string]struct{} for set.Set

Fixes #14914

[hashicorp-cla]

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