BTree index [LUCENE-5009]

[asfimport]

Currently Lucene use skip list index for better index speed. Some use case need B Tree index, such as Patent search . Documents add by day. It not need real time search , but need large volume search.

There are some B Tree index implement . such as http://algs4.cs.princeton.edu/62btrees/BTree.java.html .

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Migrated from LUCENE-5009 by Gang Luo