Avoid private nonnested classes [LUCENE-3525]

https://svn.apache.org/repos/asf/lucene/dev/trunk/lucene/src/java/org/apache/lucene/store/NativeFSLockFactory.java shows a private top-level class `NativeFSLock`. This is a JDK 1.0-ism which should be considered deprecated. Either use a nested class, or create a separate source file.

Can produce problems for tools. Cause of: https://netbeans.org/bugzilla/show_bug.cgi?id=204039

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Migrated from [LUCENE-3525](https://issues.apache.org/jira/browse/LUCENE-3525) by Jesse Glick, updated Mar 25 2012
Attachments: [LUCENE-3525.diff](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3525/LUCENE-3525.diff)

Start with lucene/src/java/org/apache/lucene/store/NativeFSLockFactory.java and inspect the private top-level NativeFSLock class. Review the attached LUCENE-3525.diff and decide whether the class should be nested or moved to a separate source file; done means the private top-level class is no longer present and the NetBeans tooling problem is addressed.

Written by the indexing model from the issue text.