Enable copying of Lucene index to local file system for read queries

Currently the Lucene index is stored in NodeStore and is accessed via OakDirectory which is an implementation of Lucene Directory. This would work fine for SegmentStore but does not work very well when BlobStore is used as with DocumentNodeStore

For such scenarios it should be possible to copy the Lucene index to local file system and then read queries are performed using that. IndexWriter would continue to use the OakDirectory to store the updated index content in NodeStore.

As Lucene does not modify any index file once created (ther are either deleted or new one created), it should be possible to incrementally update the index by copying newly created segment files.