Migrate v1 tracker saver to persistence.LocalNamedSaver

[stephen-soltesz]

This change migrates the v1 tracker saver struct to use the persistence.LocalNamedSaver, and eliminates the tracker.LocalSaver implementation. This change is in preparation for adding a v2 saver structure to natively support the separate jobs and statuses maps that the existing v1 saver struct cannot support as we migrate to more complex configurations needed for https://github.com/m-lab/etl-gardener/issues/349

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 3509

• 42 of 42 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 74.105%

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Totals
Change from base Build 3443:	0.3%
Covered Lines:	1531
Relevant Lines:	2066

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💛 - Coveralls