Purge out-dated head chains on chain completion (#1538)

## Description

There can be many head chains queued up to complete. Currently we try and process all of these to completion before we consider the node synced. 

In a chaotic network, there can be many of these and processing them to completion can be very expensive and slow. This PR removes any non-syncing head chains from the queue, and re-status's the peers. If, after we have synced to head on one chain, there is still a valid head chain to download, it will be re-established once the status has been returned. 

This should assist with getting nodes to sync on medalla faster.

beacon_node/network/src/sync/range_sync/chain_collection.rs

@@ -359,6 +359,22 @@ impl<T: BeaconChainTypes> ChainCollection<T> {
         self.state = head_state;
     }
 
+    /// This is called once a head chain has completed syncing. It removes all non-syncing head
+    /// chains and re-status their peers.
+    pub fn clear_head_chains(&mut self, network: &mut SyncNetworkContext<T::EthSpec>) {
+        let log_ref = &self.log;
+        self.head_chains.retain(|chain| {
+            if !chain.is_syncing()
+                {
+                debug!(log_ref, "Removing old head chain"; "start_epoch" => chain.start_epoch, "end_slot" => chain.target_head_slot);
+                chain.status_peers(network);
+                false
+            } else {
+                true
+            }
+        });
+    }
+
     /// Add a new finalized chain to the collection.
     pub fn new_finalized_chain(
         &mut self,

beacon_node/network/src/sync/range_sync/range.rs

@@ -322,6 +322,10 @@ impl<T: BeaconChainTypes> RangeSync<T> {
                         // the chain is complete, re-status it's peers and remove it
                         chain.status_peers(network);
 
+                        // Remove non-syncing head chains and re-status the peers
+                        // This removes a build-up of potentially duplicate head chains. Any
+                        // legitimate head chains will be re-established
+                        self.chains.clear_head_chains(network);
                         // update the state of the collection
                         self.chains.update(network);
                         // update the global state and log any change