OPENDJ-1602 New pluggable storage based backend

Index.java:
Extracted a method + moved it to VLVIndex.BufferedIndexValues.

VLVIndex.java:
Extracted a method + moved it to VLVIndex.BufferedVLVValues.
Extracted methods getVLVIndex(), isSortAttributeModified() (made the latter more efficient by exiting faster in from inner loop).
Used shouldInclude() throughout the class.
Code cleanup.

IndexBuffer.java:
Moved some methods to BufferedIndexValues and BufferedVLVValues.