Simulator and attestation service fixes (#1747)

## Issue Addressed

#1729 #1730 

Which issue # does this PR address?

## Proposed Changes

1. Fixes a bug in the simulator where nodes can't find each other due to 0 udp ports in their enr.
2. Fixes bugs in attestation service where we are unsubscribing from a subnet prematurely.

More testing is needed for attestation service fixes.

beacon_node/eth2_libp2p/src/discovery/subnet_predicate.rs

@@ -1,6 +1,6 @@
 ///! The subnet predicate used for searching for a particular subnet.
 use super::*;
-use slog::{debug, trace};
+use slog::trace;
 use std::ops::Deref;
 
 /// Returns the predicate for a given subnet.
@@ -38,7 +38,7 @@ where
                 );
                 return false;
             } else {
-                debug!(
+                trace!(
                    log_clone,
                    "Peer found on desired subnet(s)";
                    "peer_id" => format!("{}", enr.peer_id()),

beacon_node/eth2_libp2p/src/peer_manager/mod.rs

@@ -250,7 +250,9 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
             .collect();
 
         // request the subnet query from discovery
-        self.discovery.discover_subnet_peers(filtered);
+        if !filtered.is_empty() {
+            self.discovery.discover_subnet_peers(filtered);
+        }
     }
 
     /// A STATUS message has been received from a peer. This resets the status timer.