Driver should detect mixed version clusters (during upgrade) and use appropriate protocol version

Initial description:

During an upgrade from C* 2.1 to C* 3.0, it was necessary to both utilize the newest driver as well as fix the protocol version to the prior version during the upgrade.

Shouldn't the driver just detect the mixed mode cluster and utilize the appropriate protocol version?

Resolution:
Once the control connection is connected to the initial contact point, the driver checks the version of each node (system.peers.release_version) and computes the highest common protocol version. If it is lower than the current version, we downgrade and force a reconnect of the control connection before proceeding.