Heavily revise the implementation of EventLoopConnectionPool #196

Job	Run time
unit-tests / api-breakage	2m 12s
unit-tests / linux-unit (swift:5.6-focal)	1m 59s
unit-tests / linux-unit (swift:5.7-jammy)	1m 56s
unit-tests / linux-unit (swift:5.8-jammy)	2m 7s
unit-tests / linux-unit (swiftlang/swift:nightly-5.9-jammy)	2m 24s
unit-tests / linux-unit (swiftlang/swift:nightly-main-jammy)	2m 41s
unit-tests / code-coverage	1m 50s
unit-tests / gh-codeql (ubuntu-latest)	3m 8s
unit-tests / gh-codeql (macos-13)	6m 57s
unit-tests / macos-unit (macos-13, ~14.3)	2m 52s
unit-tests / macos-unit (macos-13, 15.0-beta)	3m 7s
	31m 13s

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