POC use ent as dbv2

[zwpaper]

I have divided the PR into 3 commits to show how ent could be used.

1. c7648670c839539ef94a4af9d15546de34929aa4 define the db schema, memo, memo_relation
2. 2d4ae64791340180034b7d817a54a89e6484daeb ent auto-generate the db client codes
3. ef5b193e682f13970fdcbe114550bbda9bab8816 add a dbv2 to store and use it for get memo and memo relations.

[zwpaper]

I mark it as draft, and we can discuss it here.

the PR should actually work for API v2 to get memos.

[boojack]

Sorry for the late review. Overall, I think it is a good try!