2020-08-14 Alibaba Cloud.

* version: 2.4.10
* IMPROVE: modify the logic of transaction merging in change stream mode
to improve synchronization performance. #370
* BUGFIX: fix bug of "panic with session already closed" in StartIndexSync
function.
* BUGFIX: remove autoIndexId when autoIndexId and idIndex co-exists in
create collection oplog. #400
* IMPROVE: when meets DDL in sharding, do not crash when fetch_method is
change_stream.
* IMPROVE: when fetch_method is change_stream, do not set worker thread
to MongoD number when source side is sharding. 

1. 优化change stream模式下，事务合并的逻辑，提升同步的性能。#370
2. 当autoIndexId和idIndex共存是，去除autoIndexId字段。#400
3. 当fetch_method=change_stream，遇到DDL不再直接crash。
4. 当fetch_method=change_stream，worker的个数不再强制设定为源端mongod的个数。