Fix broken tests due to missing argument in nodes.NewManager()

The coordinator test currently fails to compile as two branches that have been recently merged were intersecting. While 999c0391 (Add histogram to keep track of node healthcheck latency, 2020-03-13) introduced a new argument to nodes.NewManager(), commit 41304684 (Add correlation ID to replication jobs, 2020-03-18) added a new test that uses nodes.NewManager() with the old signature.

Fix the issue by adding the missing argument.

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The Mgr struct in praefect/nodes has an unused field for the note latency histogram. As we're keeping track of it for each connection status already, this field is never used.

Remove it to fix the linter complaining.