Discover query grouping (#1364)

## Issue Addressed

#1281

## Proposed Changes

Groups queries for specific subnets into groups of up to 3.

## Additional Info

beacon_node/eth2_libp2p/src/discovery/subnet_predicate.rs

@@ -1,9 +1,10 @@
 ///! The subnet predicate used for searching for a particular subnet.
 use super::*;
+use std::ops::Deref;
 
 /// Returns the predicate for a given subnet.
 pub fn subnet_predicate<TSpec>(
-    subnet_id: SubnetId,
+    subnet_ids: Vec<SubnetId>,
     log: &slog::Logger,
 ) -> impl Fn(&Enr) -> bool + Send
 where
@@ -23,10 +24,27 @@ where
                 }
             };
 
-            return bitfield.get(*subnet_id as usize).unwrap_or_else(|_| {
-                                   debug!(log_clone, "Peer found but not on desired subnet"; "peer_id" => format!("{}", enr.peer_id()));
-                false
-            });
+            let matches: Vec<&SubnetId> = subnet_ids
+                .iter()
+                .filter(|id| bitfield.get(**id.deref() as usize).unwrap_or(false))
+                .collect();
+
+            if matches.is_empty() {
+                debug!(
+                    log_clone,
+                    "Peer found but not on any of the desired subnets";
+                    "peer_id" => format!("{}", enr.peer_id())
+                );
+                return false;
+            } else {
+                debug!(
+                   log_clone,
+                   "Peer found on desired subnet(s)";
+                   "peer_id" => format!("{}", enr.peer_id()),
+                   "subnets" => format!("{:?}", matches.as_slice())
+                );
+                return true;
+            }
         }
         false
     }