SortingAtomicReader should be package private [LUCENE-5498]

The intended purpose of this reader is to allow you to sort your entire index with IW.addIndexes(IR).

Perhaps we should supply some kind of "tool" to do this and hide the reader. Its scary to think of someone using this for searching (based on its name and docs, its probably not clear that it would be ridiculously slow)

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Migrated from [LUCENE-5498](https://issues.apache.org/jira/browse/LUCENE-5498) by Robert Muir (@rmuir)

Start by locating SortingAtomicReader and its uses, especially the IW.addIndexes(IR) entry point described in the issue. Review its public documentation and callers to determine the visibility change; done means the reader is package private while the intended whole-index sorting path remains usable.

Written by the indexing model from the issue text.