optimization: when sorting by field, if index has one segment and field values are not needed, do not load String[] into field cache [LUCENE-2335]

Spinoff from java-dev thread "Sorting with little memory: A suggestion", started by Toke Eskildsen.

When sorting by SortField.STRING we currently ask FieldCache for a StringIndex on that field.

This can consumes tons of RAM, when the values are mostly unique (eg a title field), as it populates both int[] ords as well as String[] values.

But, if the index is only one segment, and the search sets fillFields=false, we don't need the String[] values, just the int[] ords. If the app needs to show the fields it can pull them (for the 1 page) from stored fields.

This can be a potent optimization – alot of RAM saved – for optimized indexes.

When fixing this we must take care to share the int[] ords if some queries do fillFields=true and some =false... ie, FieldCache will be called twice and it should share the int[] ords across those invocations.

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Migrated from [LUCENE-2335](https://issues.apache.org/jira/browse/LUCENE-2335) by Michael McCandless (@mikemccand), updated May 09 2016

Start by tracing sorting with SortField.STRING through FieldCache, comparing the fillFields=false and fillFields=true paths for a single-segment index. Inspect how StringIndex data is cached and shared across repeated calls. Done means the no-field-values path avoids loading unnecessary String[] values while continuing to share int[] ords safely.

Written by the indexing model from the issue text.