asyncio support?

Any thoughts on supporting asyncio? Anyone working on this?

I think we could use https://github.com/hubo1016/aiogrpc to provide an asyncio version of this module.

It doesn't necessarily have to be a fork, maybe we can create a `etcd3.aio` submodule or subpackage, containing async versions of the same interfaces.

Any thoughts?

Start by reviewing the existing synchronous interfaces in python-etcd3 and the proposed aiogrpc dependency. Compare the options of an etcd3.aio submodule or subpackage with a separate implementation, then define which interfaces and behavior an asyncio version must cover before implementation begins.

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