fix for issue #3659 by using read and write lock on index store
refactor for issue #3659 by reordering methods have similar logic as closer to themselves
checkstyle fix for issue #3659
second fix attempt for issue #3659
refactor of second fix attempt for issue #3659
3th fix attempt for issue #3659 by using "java.util.concurrent.locks.ReentrantReadWriteLock"
refactor for issue #3659 by reordering methods have similar logic as closer to themselves
enhancement for issue #3659 by using "ConcurrentSkipListMap" instead of using "SortedSet" for keys and "ConcurrentMap" for values on SortedIndexStore
checkstyle fix for issue #3659
fix for issue #3659 by ignoring null values at comparable searches on UnsortedIndexStore
unit test for issue #3659 about ignoring null values at comparable searches on SortedIndexStore and UnsortedIndexStore
Merge pull request #3745 from serkan-ozal/fix/master/issue-3659

3rd attempt for Fix/master/issue 3659