Adopt strict concurrency and Sendable in NIOConcurrencyHelpers #313

Job	Run time
Soundness / API breakage check	2m 29s
Soundness / Format check	47s
Soundness / Documentation check	2m 1s
Soundness / Broken symlinks check	4s
Soundness / License headers check	7s
Soundness / Shell check	5s
Soundness / Unacceptable language check	3s
Swift 6 Language Mode / Swift 6 language mode	1m 1s
Integration Tests / Linux (5.8)	12m 55s
Integration Tests / Linux (5.9)	14m 29s
Integration Tests / Linux (5.10)	14m 8s
Integration Tests / Linux (nightly-6.0)	17m 8s
Integration Tests / Linux (nightly-main)	17m 13s
Benchmarks / Benchmarks / Linux (5.8)	2m 35s
Benchmarks / Benchmarks / Linux (5.9)	3m 45s
Benchmarks / Benchmarks / Linux (5.10)	3m 31s
Benchmarks / Benchmarks / Linux (nightly-6.0)	4m 26s
Cxx interop / Cxx interop / Linux (5.8)	1m 3s
Benchmarks / Benchmarks / Linux (nightly-main)	5m 6s
Cxx interop / Cxx interop / Linux (5.9)	1m 14s
Cxx interop / Cxx interop / Linux (5.10)	1m 14s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 48s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 43s
Unit tests / Unit tests / Linux (5.8)	3m 14s
Unit tests / Unit tests / Linux (5.9)	3m 21s
Unit tests / Unit tests / Linux (5.10)	3m 27s
Unit tests / Unit tests / Linux (nightly-6.0)	4m 25s
Unit tests / Unit tests / Linux (nightly-main)	4m 15s
	2h 7m 37s

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