Split the stages of work with the `Client`

[cthulhu-rider]

In current Client implementation connection is opened once during first Init call. I propose to highlight the working method:

func (x *Client) Dial(PrmDial) error

The method should be called before the work. This method is going to be called during the application start-up stage.

[fyrchik]

Init already opens the connection, is this task just about changing the name and signature?

[carpawell]

Init already opens the connection

In the current implementation Init requires specifying a service name and an RPC name to open a stream, so it could not be used as "general initialization". I think that issue requires more deep refactoring.

BTW, I think that issue is quite actual. Currently, SDK client does a really strange thing to open the connection:

1. It does nothing on dialing errors (are just ignored);
2. i forget about that call and spent some time figuring out why i am facing strange requests on the server-side with zero version and empty request body.

[roman-khimov]

It's basically about making createGRPCClient() a Dial(). Which would be a nice enhancement. Luckily current SDK code hides this, so it's not a priority.