Make all resourceDescriptions of indexinputs consistent [LUCENE-5726]

This was weakly tested in a roundabout way by an assert that tried to read a lucene 2.x index.

We should try to make these consistent, on the other hand we should be careful about keeping the overhead of slice() relatively low (it would be nice to use this api in more places).

And such things should be tested elsewhere, e.g. BaseDirectoryTestCase

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Migrated from [LUCENE-5726](https://issues.apache.org/jira/browse/LUCENE-5726) by Robert Muir (@rmuir)

Start by locating the index input implementations that expose resourceDescriptions and use slice(), then inspect BaseDirectoryTestCase for the suggested coverage. Compare the resource descriptions across implementations and add tests showing consistent behavior while preserving the stated low overhead of slice().

Written by the indexing model from the issue text.