Primary key of the item where the scan operation stopped. Provide this value
in a subsequent scan operation to continue the operation from that point.
The LastEvaluatedKey is null when the entire scan result set is
complete (i.e. the operation processed the "last page").
Number of items in the complete scan before any filters are applied. A high
ScannedCount value with few, or no, Count results
indicates an inefficient Scan operation.