Maintain trusted peers (#4159)

## Issue Addressed
#4150 

## Proposed Changes

Maintain trusted peers in the pruning logic. ~~In principle the changes here are not necessary as a trusted peer has a max score (100) and all other peers can have at most 0 (because we don't implement positive scores). This means that we should never prune trusted peers unless we have more trusted peers than the target peer count.~~

This change shifts this logic to explicitly never prune trusted peers which I expect is the intuitive behaviour. 

~~I suspect the issue in #4150 arises when a trusted peer disconnects from us for one reason or another and then we remove that peer from our peerdb as it becomes stale. When it re-connects at some large time later, it is no longer a trusted peer.~~

Currently we do disconnect trusted peers, and this PR corrects this to maintain trusted peers in the pruning logic.

As suggested in #4150 we maintain trusted peers in the db and thus we remember them even if they disconnect from us.

beacon_node/network/src/sync/block_lookups/tests.rs

@@ -50,7 +50,7 @@ impl TestRig {
         };
         let bl = BlockLookups::new(log.new(slog::o!("component" => "block_lookups")));
         let cx = {
-            let globals = Arc::new(NetworkGlobals::new_test_globals(&log));
+            let globals = Arc::new(NetworkGlobals::new_test_globals(Vec::new(), &log));
             SyncNetworkContext::new(
                 network_tx,
                 globals,

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

@@ -599,7 +599,7 @@ mod tests {
             log.new(o!("component" => "range")),
         );
         let (network_tx, network_rx) = mpsc::unbounded_channel();
-        let globals = Arc::new(NetworkGlobals::new_test_globals(&log));
+        let globals = Arc::new(NetworkGlobals::new_test_globals(Vec::new(), &log));
         let cx = SyncNetworkContext::new(
             network_tx,
             globals.clone(),