Optimize some kinds of load jobs (#1762)

1. Support specifying label to Insert Into stmt.

    INSERT INTO tbl1 WITH LABEL label1 ...;

2. Return job' state corresponding to the existing label in result of stream load.

    ...
    "Status": "Label Already Exists",
    "ExistingJobStatus": "FINISHED"
    ...

3. Return the recent 2000 transactions in SHOW PROC '/transactions'

be/src/runtime/stream_load/stream_load_context.cpp

@@ -43,6 +43,8 @@ std::string StreamLoadContext::to_json() const {
         break;
     case TStatusCode::LABEL_ALREADY_EXISTS:
         writer.String("Label Already Exists");
+        writer.Key("ExistingJobStatus");
+        writer.String(existing_job_status.c_str());
         break;
     default:
         writer.String("Fail");