add sort comparator that uses OrdinalMap [LUCENE-4969]

OrdinalMap (LUCENE-4795, SOLR-4490) lets you trade off reopen time for faster faceting: it maps per-segment ordinals to a "global" space based on deviation from expected deltas.

We could add a sort comparator to the sandbox or similar that uses this, especially in the case you are using this for faceting anyway: you can then safely offload all terms bytes to disk (DiskDV) and still have performant sorting and faceting.

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Migrated from [LUCENE-4969](https://issues.apache.org/jira/browse/LUCENE-4969) by Robert Muir (@rmuir)

Start by reading OrdinalMap and the sandbox area for existing sort comparators. Trace how OrdinalMap supports faceting and how DiskDV stores term bytes, then identify the integration points for sorting. Done means a comparator can use the ordinal map while retaining performant sorting and faceting with terms bytes on disk.

Written by the indexing model from the issue text.