feature: support etcd v3

[membphis]

related issues:

https://github.com/apache/incubator-apisix/issues?q=is%3Aissue+is%3Aopen+v3+

lua-resty-etcd already supports etcd v3, we can support it by this feature:

https://github.com/api7/lua-resty-etcd/blob/master/api_v3.md

[shenal]

Current data access logic does not encapsulate the internal fields set by etcd. While we tackle the v3 upgrade we should have a separate data access layer which abstracts the internal etcd API requirements and act purely as a K/V layer. This will not only help us tackle similar upgrades but also will enable us easily allow different datastores without changes to the business logic

[shenal]

@membphis I would like to start work on this. If no one is planning to pick this up

[membphis]

that is great, I have assigned to you ^_^

[nic-chen]

@shenal @membphis
support etcd v3 is a task of summer2020( #1542 ), I think @Yiyiyimu is working on it already. You could give your advice to him, and help review when he submit pr : )

@Yiyiyimu What is your progress now ? Maybe you could submit a draft pr So we could see your progress.

[shenal]

@nic-chen Perfect !! :)

@Yiyiyimu Let me know how I can help

[Yiyiyimu]

Hi @shenal thank you for your help! I'll submit the draft pr these days and I'll let you know whenever a discussion is needed~

[spacewander]

Solved.