SCAN fix

Improve Scan operation reliability (#220)

* Initial POC fix

* Cleanup
- TokenPoolTopology has a map of token to host instead of a list of objects
- if localRack is not specified, getConnectionsToRing will get connections to a random rack
- Added exception handling to ScanTest in demo App.

* Fix build break

* More cleaner interface
- split cursorBasedResult into TokenRackMapper to seperate out writable functions

* code review comments