An old TODO points out that term is duplicated between ReplSetHeartbeatResponse._configTerm and ReplSetConfig._term. ReplSetHeartbeatResponse._config is a ReplSetConfig, so we don't need to store ReplSetHeartbeatResponse_term separately. The same is true of ReplSetHeartbeatResponse._version.

If we change this, then there may be backwards compatibility issues to work around where the fields are expected to exist in the old version and are not present in the new version.