Range scanner which iterates over ranges and adds them to queues.

Next step will be implementation of rangeIterator, integration with
Store, and creation of first actual rangeQueue implementation. Will
probably do garbage collection.
Addressed Ben's feedback; added an empty iteration unittest.
Also fixed data race in the unittest.
move scan interval to a default value and command line flag in store.go
Merge pull request #216 from cockroachdb/spencerkimball/range-scanner

Range scanner which iterates over ranges and adds them to queues.