Support specifying label in insert stmt

[morningman]

In the current implementation, the user cannot specify a Label for the INSERT command, which can lead to some usage problems.

When the user executes an INSERT command, but for some reason the connection is disconnected, the user will not know if the INSERT command is successful.

So I think we should support specifying label in insert stmt, not just using a UUID automatically
assigned by system. Because this UUID can only be returned when the statement return successfully.

The same problem occurs in the stream load job. The problem with Stream load is that although users can specify Label, they cannot view the history of stream load jobs through label. So when the connection is interrupted during the execution of Stream load, the user can not know the situation of this load job.

So, in the result of stream load import return, if it is a "Label Already Exists" error, it will return the status of the load job corresponding to this label, RUNNING or FINISHED.