gRPC client load-balancing API is ugly as f:
1. It is configured by pre-registering a balancer and the providing JSON
configuration.
2. It doesn't allow different credentials for different endpoints
(consider using "insecure" localhost and external endpoint).
3. To support frostfs usecase we also need to implement a resolver,
which has its own difficulties.
4. https://github.com/grpc/grpc-go/issues/239#issuecomment-264548415
Using interface in place of grpc.ClientConn allows us to provide custom
implentation for it (load-balancing, circuit breaker etc.).
Refs TrueCloudLab/frostfs-node#1268
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
Implement `Client.Conn` which returns the connection of the underlying
transport client. The method is going to be used for forwarding the
connection to superior clients.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
