No functional changes, just cleaning up deprecated usages that are
removed in v2 and replace one call of .Slice with .ForEach to avoid
making the intermediate copy.
This PR implements support for check_restart for checks registered
in the Nomad service provider.
Unlike Consul, Nomad service checks never report a "warning" status,
and so the check_restart.ignore_warnings configuration is not valid
for Nomad service checks.
This PR refactors agent/consul/check_watcher into client/serviceregistration,
and abstracts away the Consul-specific check lookups.
In doing so we should be able to reuse the existing check watcher logic for
also watching NSD checks in a followup PR.
A chunk of consul/unit_test.go is removed - we'll cover that in e2e tests
in a follow PR if needed. In the long run I'd like to remove this whole file.
