Don't allow IndexWriterConfig setters to chain [LUCENE-3756]

Spinoff from #4810.

I don't like that IndexWriterConfig's setters are chainable; it
results in code in our tests like this:

```
IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer(random)).setMaxBufferedDocs(2).setMergePolicy(newLogMergePolicy()));
```

I think in general we should avoid chaining since it encourages hard
to read code (code is already hard enough to read!).

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Migrated from [LUCENE-3756](https://issues.apache.org/jira/browse/LUCENE-3756) by Michael McCandless (@mikemccand), updated Feb 10 2012