Feature Request：以更直观的 Concurrent 来表达并行

[speedoops]

现在的并发是如下写法，其中用数字0、1、2……来表达顺序依赖：

	saga := dtmcli.NewSaga(DtmServer, shortuuid.New()).
		Add(Busi+"/CanRollback1", Busi+"/CanRollback1Revert", req).
		Add(Busi+"/CanRollback2", Busi+"/CanRollback2Revert", req).
		Add(Busi+"/UnRollback1", "", req).
		Add(Busi+"/UnRollback2", "", req).
		EnableConcurrent().
		AddBranchOrder(2, []int{0, 1}). // 指定step 2，需要在0，1完成后执行
		AddBranchOrder(3, []int{0, 1}) // 指定step 3，需要在0，1完成后执行

期望支持如下表达式，以更直观的 Concurrent 来表达并行：

	saga := dtmcli.NewSaga(DtmServer, shortuuid.New()).
		Concurrent(
			Add(Busi+"/CanRollback1", Busi+"/CanRollback1Revert", req),
			Add(Busi+"/CanRollback2", Busi+"/CanRollback2Revert", req)
		).
		Add(Busi+"/UnRollback1", "", req).
		Add(Busi+"/UnRollback2", "", req)

[yedf2]

欢迎pr