Fix race condition panic in loadbalancer.nextServer #1798

Job	Run time
build / Build	6m 33s
Integration Tests (certrotation)	1m 46s
Integration Tests (etcdrestore)	1m 54s
Integration Tests (localstorage)	1m 34s
Integration Tests (startup)	7m 13s
Integration Tests (custometcdargs)	45s
Integration Tests (etcdsnapshot)	8m 11s
Integration Tests (kubeflags)	7m 17s
Integration Tests (longhorn)	2m 44s
Integration Tests (secretsencryption)	2m 0s
Integration Tests (flannelnone)	1m 26s
	41m 23s

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