Clean up logs (#1541)

## Description

This PR improves some logging for the end-user. 

It downgrades some warning logs and removes the slots per second sync speed if we are syncing and the speed is 0. This is likely because we are syncing from a finalised checkpoint and the head doesn't change.

beacon_node/network/src/beacon_processor/chain_segment.rs

@@ -81,7 +81,7 @@ pub fn handle_chain_segment<T: BeaconChainTypes>(
             // reverse
             match process_blocks(chain, downloaded_blocks.iter().rev(), &log) {
                 (_, Err(e)) => {
-                    warn!(log, "Parent lookup failed"; "last_peer_id" => format!("{}", peer_id), "error" => e);
+                    debug!(log, "Parent lookup failed"; "last_peer_id" => format!("{}", peer_id), "error" => e);
                     sync_send
                         .send(SyncMessage::ParentLookupFailed{peer_id, chain_head})
                         .unwrap_or_else(|_| {

beacon_node/network/src/beacon_processor/mod.rs

@@ -774,6 +774,13 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                                     );
                                     return;
                                 }
+                                Err(BlockError::BlockIsAlreadyKnown) => {
+                                    debug!(
+                                        log,
+                                        "Gossip block is already known";
+                                    );
+                                    return;
+                                }
                                 Err(e) => {
                                     warn!(
                                         log,