4.x: Don't use overly broad SELECTs when checking schema agreement #468

When checking schema agreement driver uses SELECT * FROM system.peers query.

It looks like the only use of columns other than schema_version is calling isPeerValid function.
As far as I can tell it is still not necessary to select everything - isPeerValid needs host_id and peer, and also calls PeerRowValidator::isValid.

Is the call to PeerRowValidator::isValid necessary for checking schema agreement? I don't know, my suspicion is that it is not.

Even if this call is necessary, there are columns that are not used even by this validation and thus can surely be not selected:

• preferred_ip
• release_version
• supported_features (this is an important one because it contains a lot of data)

Not selecting tokens would be even better (because it contains even more data than supported_features), and this column is surely not needed to check schema agreement.