Support concurrency API

[honglei]

API docs：
https://etcd.io/docs/v3.4.0/dev-guide/api_concurrency_reference_v3/

[martyanov]

@honglei many thanks for the report.

Concurrency API is great. Unfortunately, the current state of aetcd3 is far from complete, it's more of a POC than a reliable solution. And now, when vanilla gRPC implementation finally got asyncio support, I think the library needs another major rewrite. :)

Leave the issue as open for now.

[stalkerg]

@martyanov is somebody working on such refactoring?

[martyanov]

@stalkerg vanilla gRPC backend is already implemented. Regarding concurrency API there is no ETA.