NIFI-305: Refactoring base class from MergeContent to allow more binning processors

[gresockj]

Pulled out the binning logic from MergeContent into a superclass for binning processors. Basically all of the min/max properties were moved to BinFiles. Mostly focused on ensuring that the functionality was preserved from MergeContent, but there is certainly room for improvement in producing a more elegantly extensible class.

[asfbot]

Joe Gresock on dev@nifi.incubator.apache.org replies:
Mark, thanks for the feedback. Good call with the final methods -- I'll
submit another commit with those changes. As for your second point, it
sounds like a good path forward -- I'll leave it to you to cherry-pick and
place appropriately in the project hierarchy.
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