Ban a fraud peer which claims a fake height and total difficulty

[garyyu]

Fix for #2 problem: #2233 (comment)

No.2 - If a new node is syncing but with this fraud peer connected, it will stuck at Header Syncing stage for ever.

[garyyu]

The banned state can't save into the database, that's driving me crazy!

	/// Ban a peer, disconnecting it if we're currently connected
	pub fn ban_peer(&self, peer_addr: &SocketAddr, ban_reason: ReasonForBan) {
		if let Err(e) = self.update_state(*peer_addr, State::Banned) {
			error!("Couldn't ban {}: {:?}", peer_addr, e);
		}

This ban_peer function is called successfully, but the banned peer always can connect again.

What I have missed?

[Updated]
A related issue is found in #2241
So, this PR should be OK now, #2241 issue will be fixed in another PR.