Make it possible for NewSession not to block

[dkropachev]

Idea is the following:

1. You create cluster configuration
2. You create session of it that does not actually connects to the cluster, but rather just starting this proccess.
3. If you are trying to execute query on such session, if it is not ready yeat it will throw you an NotReady error
4. Have an API that allows you to know if session is ready to execute queries

This API will make easier to develop apps for k8s, creating session on pod initialization and report connection status via liveness and readiness probs

[knadh]

This is great. Separating initialization and actual connections solves a number of production edge cases. Couple of considerations:

1. CreateSession() won't block and fail and will try to establish a connection in the background?
2. CreateSession() won't connect initially, but only when it's used. For example, when doing a .Query()?

Either should be fine, but 2 gives fine grained control on the behaviour, where it's possible to have config to try and connect in the background immediately or to "lazy" connect only when the session is used for querying.