Fix sendability issues in the connection pool #120

Job	Run time
Cxx interop / Construct Cxx interop matrix	4s
Unit tests / Construct unit test matrix	2s
Soundness / API breakage check	4m 22s
Soundness / License headers check	3s
Soundness / Documentation check	2m 51s
Soundness / Unacceptable language check	5s
Soundness / Broken symlinks check	3s
Soundness / Format check	36s
Soundness / Python lint check	4s
Soundness / Shell check	32s
Soundness / YAML lint check	6s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 32s
Static SDK / Static SDK / Linux (main Jammy)	1m 48s
Unit tests / Unit tests / Linux (5.10)	4m 50s
Unit tests / Unit tests / Linux (6.0)	5m 15s
Unit tests / Unit tests / Linux (6.1)	5m 7s
Unit tests / Unit tests / Linux (nightly-next)	5m 53s
Unit tests / Unit tests / Linux (nightly-main)	5m 54s
Cxx interop / Cxx interop / Linux (5.10)	2m 32s
Cxx interop / Cxx interop / Linux (6.0)	2m 54s
Cxx interop / Cxx interop / Linux (6.1)	2m 55s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 36s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 47s
	56m 51s

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