Set JobWithTarget.ID for historical and yesterday NextJob paths

[stephen-soltesz]

This change ensures that the JobWithTarget.ID is set for jobs created by the historical as well as the yesterday job sources. This change adds unit testing that covers both paths.

This change also includes small fixes to the README by removing an obsolete flag, and the job-service_test.go by removing a dependency on datastore, allowing the unit test to be run without "integration" support.

Part of:

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

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This change is 

[coveralls]

Coverage decreased (-0.08%) to 72.463% when pulling bed28c6a8e21d6c5d328a96e43dba5a895bc1501 on sandbox-soltesz-fix-job-id into f6a4eced2192afce7f8feef735afcf469d15fce0 on master.

[SaiedKazemi]

[stephen-soltesz]

Fast! Thank you!