push down filtering by locus into a predicate

[timodonnell]

When calling variants at only a subset of loci, it would be great if we could push that into an ADAM predicate to avoid loading the whole dataset. This would mean writing a predicate that takes a LociSet and a window size, and filters overlapping reads. It would also involve some refactoring, including the loadReads() and loci() functions in Common, since right now the filtering is done separately after loading.

[arahuja]

We can do this if we are loading ADAM reads or some other parquet serialized data. Thoughts are on our approach to this @ryan-williams @timodonnell