Set batch priority on dedup and join

[stephen-soltesz]

This change adds QueryConfig.Priority for Dedup and Join jobs created by gardener using "Batch" priority. This change should avoid quota limits for interactive jobs recently seen in https://github.com/m-lab/etl-gardener/issues/410.

Fixes:

• https://github.com/m-lab/etl-gardener/issues/410

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 3588

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 77.771%

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Totals
Change from base Build 3581:	0.1%
Covered Lines:	1242
Relevant Lines:	1597

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💛 - Coveralls