Don't let dbft protocol value be `unknown`

[nicolegys]

[roman-khimov]

Something like validator/protocol status (elected/not, height, view, primary/backup)?

[nicolegys]

Something like validator/protocol status (elected/not, height, view, primary/backup)?

The dbft API has provided access to the state of the dbft consensus engine, like dbft_status, dbft_getSnapshot, etc. I think the dbft field here only needs to return something brief or even can be removed, because there's also a dbft in eth field.