connection_pool: fix excessive idle CPU usage while waiting for requests #1086

Job	Run time
pack_deb	-1s
publish_deb	0s
run_tests_rpm	-1s
publish_pip	0s
publish_rpm	0s
pack_rpm	-7s
pack_pip	-8s
run_tests_pip_package_linux	-8s
run_tests_deb	-9s
run_tests_pip_package_windows	-9s
	-43s

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