fix: gc: Correctly calculate lookback (#418) #449

Job	Run time
ci-lint	18s
build-2k	4m 25s
build-debug	4m 16s
build-calibnet	4m 13s
build-forest	4m 17s
build-mainnet	4m 38s
gen-check	9m 25s
gofmt	18s
lint	3m 30s
mod-tidy-check	30s
setup-params	3s
test (test-itest-curio, ./itests/curio_test.go)	7m 10s
test (test-all, `go list ./... \| grep -v curio/itests`)	2m 58s
test (test-itest-harmonyDB, ./itests/harmonydb_test.go)	2m 53s
test (test-itest-alertnow, ./itests/alertnow_test.go)	1m 35s
	50m 29s

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