LPS-44143 Add support of IndexSearcherManager, to share IndexReader accross multi threads.
LPS-44143 Refactor IndexAccessor and IndexSearcherManager
LPS-44143 Pull up IndexSearcherManager as a standalone class
LPS-44143 Add IndexSearcherManagerTest.
LPS-44143 Refactor test
LPS-44143 Make sure all index changes trigger IndexSearcherManager.invalid()
LPS-44143 Add support of IndexSearcherManager in LuceneHelper
LPS-44143 Apply to LuceneHelper's usage
LPS-44143 Fix test
LPS-44143 Deperate methods.