[GOBBLIN-492] Refactor LoopingDatasetFinderSource to make it extensible

[autumnust]

Dear Gobblin maintainers,

Some changes to make DeepIterator extensible and easily being overwritten by different implementation of iterators. They includes:

• Several signature change.
• Create getWorkUnitIterator method to be overwritten in subclass.
• Create doComputeNext method to separate the logic of getting next available dataset object (Can be a dataset or a partition) from DeepIterator itself, so that different iterator that extends it can inherit the existing functionality of looping datasets, watermarks, etc. but have its own logic on how to fetch the next available dataset.

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-492] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-492

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Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
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  4. Subject uses the imperative mood ("add", not "adding")
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[ibuenros]

+1 Looks good other than one comment.

[ibuenros]

Why does this need to move inside the method?