KafkaController的状态机和partition leader选择器

Partition：Topic的分区
Partition状态机

Replica：存放Partition复制品的broker（kafka server实例）
Replica状态机

为partition选举新leader的4种选择器

ControlledShutdownLeaderSelector

OfflinePartitionLeaderSelector

PreferredReplicaPartitionLeaderSelector

ReassignedPartitionLeaderSelector