cluster discovery: expose a last discovery result

[Totktonada]

I suspect that there are cases when knowledge of actual cluster discovery result may be helpful. Now the only way to get it is to inherit its own cluster client from TarantoolClusterClient, overrride onInstancesRefreshed() method (don't forget to call super.onInstancesRefreshed()) and store addresses here. This does not look conveniently.

This idea appears during discussion of reconnection abilities. Cite from #228:

Re-creation of a client can lead to inability to connect if a user bootstraps the client from one instance and use cluster discovery to fetch others if the need to re-create the client occurs during a problem with the initial instance (or if the initial configuration was not updated at time). I think it worth to expose last cluster discovery result to give a user an ability to handle this problem if it want to re-create a cluster client anyway.