improve getLiveDocs() performance [LUCENE-3500]

On segmentReader, liveDocs is volatile because they used to be loaded on demand.
This no longer appears to be the case... liveDocs are always loaded up front.
This also means that getLiveDocs() can never fail (even after close), and we can remove the ensureOpen call.
Minor optimizations, but volatile reads still do prevent optimizations across that boundary.

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Migrated from [LUCENE-3500](https://issues.apache.org/jira/browse/LUCENE-3500) by Yonik Seeley (@yonik)
Attachments: [LUCENE-3500.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3500/LUCENE-3500.patch)

Start at SegmentReader's liveDocs field and getLiveDocs() method, then inspect how liveDocs is initialized and whether it is always available before access. The work is done when the unnecessary volatile and ensureOpen behavior described in the issue are removed without changing live-docs behavior, including access after close.

Written by the indexing model from the issue text.