update README for null primary index

Summary: Ref T2595

Test Plan: N/A

Reviewers: linshangquan, heliangliang

Reviewed By: heliangliang

Subscribers: linshangquan, heliangliang

Maniphest Tasks: T2595

Differential Revision: https://phabricator.d.xiaomi.net/D8131
update thrift files

Summary: Ref T2621

Test Plan: N/A

Reviewers: linshangquan, heliangliang

Reviewed By: heliangliang

Subscribers: linshangquan, heliangliang

Maniphest Tasks: T2621

Differential Revision: https://phabricator.d.xiaomi.net/D8242
add datum map class