Consistent tracking of disconnected peers (#2650)

## Issue Addressed

N/A

## Proposed Changes

When peers switching to a disconnecting state, decrement the disconnected peers counter. This also downgrades some crit logs to errors. 

I've also added a re-sync point when peers get unbanned the disconnected peer count will match back to the number of disconnected peers if it has gone out of sync previously.

beacon_node/network/src/sync/manager.rs

@@ -672,7 +672,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
             }
             peer_info.is_connected()
         } else {
-            crit!(self.log, "Status'd peer is unknown"; "peer_id" => %peer_id);
+            error!(self.log, "Status'd peer is unknown"; "peer_id" => %peer_id);
             false
         }
     }