兩個時間戳相減

兩個時間戳相減就是相差的毫秒

{
    "task_name": "VERTEX_LABEL:1:node",
    "task_progress": 0,
    "task_create": 1577969459291,     13位，毫秒級別
    "task_status": "running",
    "task_update": 1577969459291,
    "task_retries": 0,
    "id": 8,
    "task_type": "remove_schema",
    "task_callable": "com.baidu.hugegraph.job.schema.VertexLabelRemoveCallable"
}