[AuditPlugin] Conflict load label from different FE

[morningman]

Describe the bug
the audit plugin is running on different FE, but currently the format of load label is all same, like：

audit_20200425_033418

This will cause label conflict, and you will see warn log in some be.INFO:

W0525 15:34:18.306124 54334 stream_load_executor.cpp:147] begin transaction failed, errmsg=errCode = 2, detailMessage = Label [audit_20200425_033418] has already been used.id=4c42099eea72f17b-faf98f349594f896, job_id=-1, txn_id=-1, label=audit_20200425_033418