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/lighthouse_network/src/peer_manager/peerdb.rs

@@ -1062,7 +1062,7 @@ impl<TSpec: EthSpec> PeerDB<TSpec> {
             if let Some(to_drop) = self
                 .peers
                 .iter()
-                .filter(|(_, info)| info.is_disconnected())
+                .filter(|(_, info)| info.is_disconnected() && !info.is_trusted())
                 .filter_map(|(id, info)| match info.connection_status() {
                     PeerConnectionStatus::Disconnected { since } => Some((id, since)),
                     _ => None,