MINOR: Small cleanups in `AlterIsr` handling logic (#9663)
MINOR: Small cleanups in `AlterIsr` handling logic (#9663)

A few small cleanups in `Partition` handling of `AlterIsr`:

- Factor state update and log message into `sendAlterIsrRequest`
- Ensure illegal state error gets raised if a retry fails to be enqueued
- Always check the proposed state against the current state in `handleAlterIsrResponse`
- Add `toString` implementations to `IsrState` case classes

Reviewers: Chia-Ping Tsai <51ef25fd0a3611d18ffb478ae2ae610bbbd2432c@gmail.com>
Chasego/kafka,apache/kafka,TiVo/kafka,guozhangwang/kafka,Chasego/kafka,TiVo/kafka,guozhangwang/kafka,TiVo/kafka,guozhangwang/kafka,Chasego/kafka,Chasego/kafka,guozhangwang/kafka,lindong28/kafka,apache/kafka,apache/kafka,TiVo/kafka,apache/kafka,lindong28/kafka,lindong28/kafka,lindong28/kafka