Refactor IndexingService to reference IndexProxy instances using the newly introduced IndexRef instances

o Initialize label scan store before indexing service
o Extracted concurrent map from IndexingService into type safe IndexMap class
Instantiate new OnlineLabelIndexProxy for label index refs

 o Extracted IndexKind as a top-level class
 o Made IndexDescriptor aware of label scan indexes
 o Made FailedIndexProxy work without a populator in case we don't have one