Interim commit to rename a bunch of files.
Addressed all code review comments for KeySampler and KeyVersionFetcher. Renamed many classes and methods related to FetchStreamRequestHandler.

- All sub-classes of FetchStreamRequestHandler have been renamed to have a more consistent nomenclature.
- Did some further refactoring in the FullScan* classes to move more work from leaf classes to FullScanFetchRequestHandler.java
- moved scan accounting to overall bae class
- Added getNodesPartitionIdForKey method to StoreInstance to help with some fetch logic