Feature request: grpc conn manager

[MaruHyl]

Ref: https://github.com/go-kit/kit/blob/master/examples/apigateway/main.go#L184

Based on this hint, I wrote some code and wondered if this could be contributed to the kit. Do you have time to look at it @ChrisHines ? If it is not good enough, can it be an example?
Here, https://github.com/MaruHyl/xgrpc

[MaruHyl]

@peterbourgon This is an initial version. Can you give some suggestions?

[peterbourgon]

@basvanbeek I think you are the original author of that comment, can you provide some feedback? I would have to spend some time to get up to speed to be able to comment effectively.

[basvanbeek]

I will take a look on Thursday.

[basvanbeek]

Not forgotten, simply have had no time yet. Still on my todo list.