add some method to check connection alive

[vtolstov]

For persistent connection we need to have some method to understand connection status (alive or not), may be with optional callback to ability to reconnect.

[vtolstov]

as i see, we can utilize https://github.com/eBay/go-ovn/blob/master/ovnnotify.go, may be provide some time.Duration for heartbeat between client<->server (for echo, and if it not received with needed time, invalidates conn and hook callback?)

[vtolstov]

may be pass something like this to GetInstance
type Handler struct {
Disconnected(interface{})
}

so use can handle this and reconnect?

[vtolstov]

@hzhou8 i think that we need to decide some interface how to pass disconnect handler.
I think that we need to have some Config struct with ovn endpoint address in url.URL format, may be ssl credentials like ca, cert, key. and pointer to disconnect handler.
What you think?
So to create instance we need to call c, err := ovn.Client(&Config{xxx})