Add JobIterator and DateIterators to job service

[stephen-soltesz]

This change adds three new types to eventually replace the current implementation of the job-service NextJob logic.

This change adds two types that implement a DateIterator interface:

• DailyIterator - for generating a sequence of dates for processing "yesterday".
• HistoricalIterator - for generating a sequence of dates for historical processing.

This change adds a generic JobIterator that can accept either a DailyIterator or HistoricalIterator. Ultimately, the job service will include two JobIterators using one of each DateIterator.

Notable about this implementation:

• Previously (currently) the NextJob logic follows a persistence cycle below. If restarted mid-processing, the next date is reloaded before all data is processed, potentially skipping some data.
  • load (on restart)
  • update, filter, save (for next job date).
• The JobIterator here follows a persistence cycle below. If restarted mid-processing, the current date is reloaded potentially reprocessing some data, but none is skipped.
  • load (on restart)
  • filter, save, update (for next job date).

Part of:

• https://github.com/m-lab/etl-gardener/issues/349

Tested only with unit tests. This change adds a new interface before using it.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 3432

• 83 of 83 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.02%) to 74.099%

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Totals
Change from base Build 3423:	1.02%
Covered Lines:	1625
Relevant Lines:	2193

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💛 - Coveralls

[stephen-soltesz]

Thank you!