merge-sort step cannot be cancelled if some routine failed and might oom

[D3Hunter]

Bug Report

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1. Minimal reproduce step (Required)

merge-sort step cannot be cancelled if some routine failed, we need use WithContext to create err-group

tidb/br/pkg/lightning/backend/external/merge.go

Line 38 in 9d07f83

var eg errgroup.Group

currently we calculate writer mem-limit using half of machine memory, we need limit it by concurrency

tidb/br/pkg/lightning/backend/external/merge.go

Lines 32 to 36 in 9d07f83

	memTotal, err := memory.MemTotal()
	if err != nil {
	return err
	}
	memSize := (memTotal / 2) / uint64(len(dataFilesSlice))

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)