clusterimpl: send single picker update synchronously on receipt of co… #434

Job	Run time
vet-proto	28s
tests (vet, 1.22)	2m 12s
tests (extras, 1.22)	2m 39s
tests (tests, 1.22)	3m 42s
tests (tests, 1.22, -race)	5m 31s
tests (tests, 1.22, -race -tags=buffer_pooling)	5m 26s
tests (tests, 1.22, 386)	4m 36s
tests (tests, 1.22, arm64)	6m 28s
tests (tests, 1.21)	2m 8s
tests (tests, 1.22, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	5m 36s
	38m 46s

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