Allow the SlowCompositeReaderWrapper and MultiDocValues to accept acceptableOverheadRatio for OrdinalMap [LUCENE-6101]

There are use cases where it's useful to be able to trade off performance and memory when accessing top level DocValues ordinals.

This issue adds methods to the SlowCompositeReaderWrapper and MultiDocValues that allow an acceptableOverheadRatio to be passed in and applied to the OrdinalMapping.

If people are ok with this approach I will add tests and performance benchmarks.

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Migrated from [LUCENE-6101](https://issues.apache.org/jira/browse/LUCENE-6101) by Joel Bernstein (@joel-bernstein)
Attachments: [lucene-6101.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6101/lucene-6101.patch)

Start by reviewing SlowCompositeReaderWrapper, MultiDocValues, and the attached lucene-6101.patch to understand the proposed OrdinalMap API changes. Confirm that both entry points accept acceptableOverheadRatio and apply it to ordinal mapping, then add the tests and performance benchmarks mentioned in the issue.】【。

Written by the indexing model from the issue text.