Term(Ord)ValComparator.value return BytesRefs that are re-used internally [LUCENE-6808]

While working on [SOLR-6168](https://issues.apache.org/jira/browse/SOLR-6168), which involves some non-trivial usage of `FieldComparator`, I discovered some weird bugs anytime I was using TermOrdValComparator. I ultimately tracked this down to the fact that the `BytesRef` instances returned by `TermOrdValComparator.value(int slot)` are backed by `BytesRefBuilder` instances that the Comparator hangs on to and re-uses – so the values a caller gets back from `FieldComparator.value(slot)` might be changed out from under it before it has a chance to use that value in something like `FieldComparator.compareValues(first,second)`.

The general approach when dealing with BytesRef instances (as i understand it) is that the caller is responsible for making a copy if it wants to hang on to it – but in this case that would violate the generic API of FieldComparator – callers would have to pay attention to when a `FieldComparator` is a
`FieldComparator` and do casting to copy the BytesRef.

It seems like the right solution is for `TermOrdValComparator.value(slot)` (and `TermValComparator.value(slot)` which has a similar BytesRef usage) to return `BytesRef.deepCopyOf(values\[slot\])`

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Migrated from [LUCENE-6808](https://issues.apache.org/jira/browse/LUCENE-6808) by Chris M. Hostetter (@hossman), updated Sep 22 2015
Attachments: [LUCENE-6808.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6808/LUCENE-6808.patch) (versions: 2)
Linked issues:
- [SOLR-6168](https://issues.apache.org/jira/browse/SOLR-6168)

Start by locating TermOrdValComparator.value(int slot) and TermValComparator.value(int slot), then inspect how FieldComparator.value(slot) and compareValues(first, second) use the returned BytesRef instances. The issue proposes deep-copying the slot value; done means returned values remain stable when the comparator reuses its internal BytesRefBuilder-backed values, with relevant comparator behavior covered by tests.

Written by the indexing model from the issue text.