Clean up Sendability in Thread and ThreadPool #1264

Job	Run time
Construct integration test matrix	3s
Vsock tests	2m 24s
Benchmarks / Construct Benchmarks matrix	3s
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	3s
Soundness / API breakage check	2m 6s
Soundness / Documentation check	2m 9s
Soundness / Broken symlinks check	5s
Soundness / License headers check	7s
Soundness / Unacceptable language check	5s
Soundness / Format check	48s
Soundness / Python lint check	6s
Soundness / Shell check	35s
Soundness / YAML lint check	3s
Benchmarks / Benchmarks / Linux (5.9)	5m 18s
Benchmarks / Benchmarks / Linux (5.10)	5m 5s
Benchmarks / Benchmarks / Linux (6.0)	5m 29s
Benchmarks / Benchmarks / Linux (nightly-6.1)	4m 31s
Benchmarks / Benchmarks / Linux (nightly-main)	4m 21s
Unit tests / Unit tests / Linux (5.9)	3m 56s
Unit tests / Unit tests / Linux (5.10)	3m 45s
Unit tests / Unit tests / Linux (6.0)	4m 11s
Unit tests / Unit tests / Linux (nightly-6.1)	4m 39s
Unit tests / Unit tests / Linux (nightly-main)	4m 25s
Integration tests / Linux (5.9)	23m 3s
Integration tests / Linux (5.10)	21m 26s
Integration tests / Linux (6.0)	23m 38s
Integration tests / Linux (nightly-6.1)	24m 25s
Cxx interop / Cxx interop / Linux (5.9)	1m 8s
Integration tests / Linux (nightly-main)	23m 21s
Cxx interop / Cxx interop / Linux (5.10)	1m 8s
Cxx interop / Cxx interop / Linux (6.0)	1m 17s
Cxx interop / Cxx interop / Linux (nightly-6.1)	1m 33s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 32s
	2h 56m 51s

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