Revert peer DB changes from #2724 (#2828)

## Proposed Changes

This reverts commit 53562010ec from PR #2724

Hopefully this will restore the reliability of the sync simulator.

beacon_node/lighthouse_network/src/discovery/mod.rs

@@ -679,7 +679,8 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
                 // Determine if we have sufficient peers, which may make this discovery unnecessary.
                 let peers_on_subnet = self
                     .network_globals
-                    .peers()
+                    .peers
+                    .read()
                     .good_peers_on_subnet(subnet_query.subnet)
                     .count();