NIFI-4828: Fix MergeContent to process all ready bins

[ijokarumawak]

NOTE to reviewers

This pull request has 3 commits intentionally. The first one is sufficient to address NIFI-4828. The other two commits are further refactoring those are not required, but should be incorporated, too. I didn't create dedicated JIRAs for those since they're too trivial.

Please squash commits and merge if all of those look fine. Or remove the 2nd or 3rd commit if it looks over engineered. Thank you!

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[markap14]

@ijokarumawak thanks for addressing! Code review looks good. All unit tests pass (including the ones that were @ignored previously!) and some local testing all worked out exactly as expected. +1 merged to master.