Sc store reset

The SC.Store.reset() method was not clearing out previously constructed RecordArrays, even though the underlying data hash was being cleared. While technically this is valid -> most of the storekeys are set to SC.Record.EMPTY rather than being deleted altogether, which means the storeKey lists inside of the RA's aren't invalidated -> it seems a little strange that a method labeled "reset" does not, in fact, clean up the entire store.

This patch ensures that the entire store is cleared, and adds a few unit tests that enforce the one-record-array-instance-per-query contract.