optimize DocTermOrds in cases where the underlying TermEnum being wraped supports ord() [LUCENE-6545]

Prior to #7587, DocTermOrds had an optimization when the TermEnum of the field being Uninverted already supported ord().

This optimization was removed in #7587 (see r1684704) because it was found to produce incorrect results for numeric fields that had a precisionStep.

This issue is to track the possibility of re-adding a correct version of this optimization.

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Migrated from [LUCENE-6545](https://issues.apache.org/jira/browse/LUCENE-6545) by Chris M. Hostetter (@hossman), 1 vote
Linked issues:
- #7587

Start by reviewing linked issue #7587 and change r1684704 to understand why the DocTermOrds optimization was removed and how TermEnum.ord() interacts with numeric fields using a precisionStep. Done means a corrected optimization can be evaluated without reproducing the incorrect results that prompted its removal.

Written by the indexing model from the issue text.