Need a clean way for Dir/MultiReader to "merge" the AttributeSources of the sub-readers [LUCENE-2154]

The flex API allows extensibility at the Fields/Terms/Docs/PositionsEnum levels, for a codec to set custom attrs.

But, it's currently broken for Dir/MultiReader, which must somehow share attrs across all the sub-readers. Somehow we must make a single attr source, and tell each sub-reader's enum to use that instead of creating its own. Hopefully Uwe can work some magic here :)

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Migrated from [LUCENE-2154](https://issues.apache.org/jira/browse/LUCENE-2154) by Michael McCandless (@mikemccand), updated May 09 2016
Attachments: [ASF.LICENSE.NOT.GRANTED--LUCENE-2154-Jakarta-BCEL.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2154/ASF.LICENSE.NOT.GRANTED--LUCENE-2154-Jakarta-BCEL.patch), [LUCENE-2154.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2154/LUCENE-2154.patch) (versions: 2), [LUCENE-2154-cglib.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2154/LUCENE-2154-cglib.patch), [LUCENE-2154-Jakarta-BCEL.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2154/LUCENE-2154-Jakarta-BCEL.patch), [LUCENE-2154-javassist.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2154/LUCENE-2154-javassist.patch) (versions: 2)

Start by tracing the flex API through Dir/MultiReader and the Fields, Terms, Docs, and PositionsEnum levels described in the issue. Determine how sub-readers currently create AttributeSources; done means the readers share a single attribute source without breaking custom codec attributes.

Written by the indexing model from the issue text.