Misc Peer sync info adjustments (#1896)

## Issue Addressed
#1856 

## Proposed Changes
- For clarity, the router's processor now only decides if a peer is compatible and it disconnects it or sends it to sync accordingly. No logic here regarding how useful is the peer. 
- Update peer_sync_info's rules
- Add an `IrrelevantPeer` sync status to account for incompatible peers (maybe this should be "IncompatiblePeer" now that I think about it?) this state is update upon receiving an internal goodbye in the peer manager
- Misc code cleanups
- Reduce the need to create `StatusMessage`s (and thus, `Arc` accesses )
- Add missing calls to update the global sync state

The overall effect should be:
- More peers recognized as Behind, and less as Unknown
- Peers identified as incompatible

beacon_node/eth2_libp2p/src/service.rs

@@ -249,7 +249,7 @@ impl<TSpec: EthSpec> Service<TSpec> {
         self.swarm.report_peer(peer_id, action);
     }
 
-    // Disconnect and ban a peer, providing a reason.
+    /// Disconnect and ban a peer, providing a reason.
     pub fn goodbye_peer(&mut self, peer_id: &PeerId, reason: GoodbyeReason) {
         self.swarm.goodbye_peer(peer_id, reason);
     }