scheduler: optimize scheduleCycle logic when notEnoughChildren #7609

Job	Run time
golangci-lint	58s
unit-tests(Check Go modules)	34s
unit-tests(Check manifests)	38s
unit-tests(Check auto-generated codes)	4m 35s
unit-tests(Verify gofmt)	35s
unit-tests(Verify govet)	55s
unit-tests(Run Go build)	1m 20s
unit-tests(Run Go test)	4m 44s
build-and-push (koordlet)	3m 30s
build-and-push (koord-manager)	2m 9s
build-and-push (koord-scheduler)	3m 35s
build-and-push (koord-descheduler)	2m 33s
	26m 6s

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