Try opening initial control connection to all known contact points simultaneously

[muzarski]

This is something that cpp-driver does.

The relevant code:

• opening initial connections to all resolved contact points: https://github.com/scylladb/cpp-driver/blob/master/src/cluster_connector.cpp#L165-L188 and https://github.com/scylladb/cpp-driver/blob/master/src/cluster_connector.cpp#L118-L126
• connector is notified about one of control connection being opened/broken: https://github.com/scylladb/cpp-driver/blob/master/src/cluster_connector.cpp#L190-L309

This is an optimization that we could introduce in the future. This would speed up the process of opening the session. Currently, we try opening the control connections (and fetching initial metadata) on each node sequentially.

I believe that no API breaking changes would be required, since we would only need to refactor internal API. Thus, I think this can be addressed in post 1.0 driver.

[Lorak-mmk]

Closing in favor of #1279