external engine oom on loadIngestData

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

import 32g data using global sort on a 4c8g node

backend worker concurrency is 32, range data loader concurrency is min(int(MergeSortOverlapThreshold) / len(e.dataFiles), 8)，write&ingest worker concurrency is fixed as 32, external engine might load too many data into memory

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master