Use async ZookeeperClient in Controller

Synchronous zookeeper writes means that we wait an entire round trip before doing the next write. These synchronous writes are happening at a per-partition granularity in several places, so partition-heavy clusters suffer from the controller doing many sequential round trips to zookeeper.

• PartitionStateMachine.electLeaderForPartition updates leaderAndIsr in zookeeper on transition to OnlinePartition. This gets triggered per-partition sequentially with synchronous writes during controlled shutdown of the shutting down broker's replicas for which it is the leader.
• ReplicaStateMachine updates leaderAndIsr in zookeeper on transition to OfflineReplica when calling KafkaController.removeReplicaFromIsr. This gets triggered per-partition sequentially with synchronous writes for failed or controlled shutdown brokers.

KAFKA-5501 introduced an async ZookeeperClient that encourages pipelined requests to zookeeper. We should replace ZkClient's usage with this client.