release-v2.4.0-20200326

add change stream option to fetch the incremental data.
pay attention: this version is not compatible with the former version.
主要特性：增加change stream对接接口，用户可以选择用change stream的方式进行拉取解决move chunk的问题，从而不需要关闭balancer。
注意：本版本不与之前的版本（<2.4）兼容，配置文件也需要进行升级，如果有已经存在的checkpoint也需要删除。
具体文档请关注wiki，后面会进行补充。

2020-03-25 Alibaba Cloud.

* version: 2.4.0
* IMPROVE: support change stream to solve move chunk problem.
* IMRPOVE: add feature compatiable version(fcv) for configuration and
checkpoint.
* IMPROVE: optimize orphan document filter.
* IMPROVE: block password in conf and log.
* IMPROVE: retry to insert document when duplicate error happen in full
sync stage.
* IMRPOVE: add 'full_sync.create_index' in configuration.
* DISABLE: temporarily disable oplog stored to disk during document
replication released on v2.2.1.