* Fix typo in redis-sentinel.md
* Thanks @tccbest 

update configuraiton to configuration

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*Total -- 163.03kb -> 68.60kb (57.92%)

/images/redis-single-thread-model.png -- 81.52kb -> 34.30kb (57.92%)
/docs/high-concurrency/images/redis-single-thread-model.png -- 81.52kb -> 34.30kb (57.92%)

* 更新 redis 单线程模型文章知识点描述；
* 重新绘图，更清晰直观；
* 学习的时候，参照文字描述与图片，更加容易理解。

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倒排索引的一些补充说明

推荐《大型网站技术架构_核心原理与案例分析——李智慧》

Update cobar desc

- Update RocketMQ desc
- Fix #40 @rainzhao

- Fix typo in why-cache.md
- Close #39 @jiaxingzheng

- Fix typo in redis-persistence
- Close #37

add additional method to handle distributed session

Small changes in `dubbo-load-balancing.md`

- Fix typo in distributed-session
- Close #34 @chenlang888881

- Indices created in Elasticsearch 6.0.0 or later may only contain a single mapping type. Indices created in 5.x with multiple mapping types will continue to function as before in Elasticsearch 6.x. Mapping types will be completely removed in Elasticsearch 7.0.0.
- Fix #33
- Thank you @jptangchina

- Fix #32
- Thank you @CavaliersFor

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*Total -- 56.62kb -> 24.34kb (57.01%)

/images/database-id-sequence-step.png -- 28.31kb -> 12.17kb (57.01%)
/docs/high-concurrency/images/database-id-sequence-step.png -- 28.31kb -> 12.17kb (57.01%)

更新分库分表后如何生成唯一ID

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*Total -- 46.21kb -> 20.38kb (55.89%)

/images/mq-10.png -- 23.11kb -> 10.19kb (55.89%)
/docs/high-concurrency/images/mq-10.png -- 23.11kb -> 10.19kb (55.89%)

- 修改 Kafka 图片小错误
- 调整图片展示位置