Backport #6185: optimize mempool #19968

Job	Run time
filter-changes	3s
lint	3m 8s
build (ubuntu-22.04)	49s
quicktests	1m 36s
build-tools (ubuntu-22.04)	52s
build (ubuntu-latest-arm-8-cores)	3m 23s
build-tools (ubuntu-latest-arm-8-cores)	2m 42s
unittests (ubuntu-22.04)	20m 35s
build (macos-13)	2m 11s
build-tools (macos-13)	1m 53s
build (self-hosted, macOS, ARM64, go-spacemesh)	27s
build-tools (self-hosted, macOS, ARM64, go-spacemesh)	8m 17s
unittests (ubuntu-latest-arm-8-cores)	26m 30s
build (windows-2022)	5m 47s
build-tools (windows-2022)	4m 2s
unittests (macos-13)	25m 13s
unittests (self-hosted, macOS, ARM64, go-spacemesh)	17m 46s
unittests (windows-2022)	30m 46s
ci-status	3s
	2h 36m 3s

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