Before changing default insert operation to streaming load, if the select result
of a insert stmt is empty, a label will still be returned to the user, and user
can use this label to check the insert load job's status.
After changing the insert operation, if the select result is empty, a exception
will be thrown to user client directly without any label.
This new usage pattern is not friendly to already existed users, which is forcing
them to change their way of using insert operation.
So I add a new FE config 'using_old_load_usage_pattern', default is false.
If set to true, a label will be returned to user even if the select result is empty.
bde362c3cd