Fix sendability issues in the connection pool #119

Job	Run time
Cxx interop / Construct Cxx interop matrix	5s
Unit tests / Construct unit test matrix	3s
Soundness / Unacceptable language check	2s
Soundness / Documentation check	2m 53s
Soundness / API breakage check	4m 22s
Soundness / Broken symlinks check	2s
Soundness / Format check	34s
Soundness / License headers check	3s
Soundness / Python lint check	4s
Soundness / Shell check	33s
Soundness / YAML lint check	4s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 24s
Static SDK / Static SDK / Linux (main Jammy)	2m 4s
Unit tests / Unit tests / Linux (5.10)	2m 22s
Unit tests / Unit tests / Linux (6.0)	5m 14s
Unit tests / Unit tests / Linux (6.1)	4m 57s
Unit tests / Unit tests / Linux (nightly-next)	5m 56s
Unit tests / Unit tests / Linux (nightly-main)	5m 55s
Cxx interop / Cxx interop / Linux (5.10)	2m 34s
Cxx interop / Cxx interop / Linux (6.0)	2m 50s
Cxx interop / Cxx interop / Linux (6.1)	2m 52s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 35s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 38s
	54m 6s

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