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/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