Double-check peer status after syncing to initial head.

As mentioned in ethereum/trinity#1881, we will need to double-check with peers after having imported all blocks up to the head provided in the initial status exchange.

From the spec: *Note: Under abnormal network condition or after some rounds of BeaconBlocksByRange requests, the client might need to send Status request again to learn if the peer has a higher head. Implementers are free to implement such behavior in their own way.*