RangeScan: add an optional client supplied name

Given the JSON input on create we could add an optional name field, so the client could pass a useful name. The name would not have to be unique, but just to help distinguish range scans from each other.

This could ease debug certainly of KV unit tests where a test failed and dumped the logs, but we can't see which scan related to the test. Of course the tests could also be updated to dump the scan ID, but for now a name would be a nice improvement.