Persists the targets acquired by the TargetManager

[mimir-d]

Issue by marcoguerri Thursday Feb 06, 2020 at 15:06 GMT Originally opened as https://github.com/facebookincubator/contest/issues/34

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Right now there is no source of truth that tells which targets were acquired at the beginning of the job, that can be references for example at events submission time. To have a normalized data model, we need to store targets acquired as well.