core/stats can return two different schemas in 'transferring' field.
One is object with fields the other is just plain string.
This is confusing, unnecessary and makes defining response schema
more difficult. It also returns `lastError` as value which can be
rendered differently depending on source of error.
This change standardizes 'transferring' filed to always return
object but with reduced fields if they are not available.
Former string item is converted to {name:remote_name} object.
'lastError' is forced to be a string as in some cases it can be encoded
as an object.
4ba6532915