add retry logic to peer discovery and an expiration time for peers (#1203)

* add retry logic to peer discovery and an expiration time for peers

* Restructure discovery

* Add mac build to CI

* Always return an error for Health when not linux

* Change macos workflow

* Rename macos tests

* Update DiscoverPeers messages to pass Instants. Implement PartialEq for AttServiceMessage

* update discover peer queueing to always check existing messages and extend min_ttl as necessary

* update method name and comment

* Correct merge issues

* Add subnet id check to partialeq, fix discover peer message dups

* fix discover peer message dups

* fix discover peer message dups for real this time

Co-authored-by: Age Manning <Age@AgeManning.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/eth2-libp2p/src/discovery/subnet_predicate.rs

@@ -0,0 +1,33 @@
+///! The subnet predicate used for searching for a particular subnet.
+use super::*;
+
+/// Returns the predicate for a given subnet.
+pub fn subnet_predicate<TSpec>(
+    subnet_id: SubnetId,
+    log: &slog::Logger,
+) -> impl Fn(&Enr) -> bool + Send + 'static + Clone
+where
+    TSpec: EthSpec,
+{
+    let log_clone = log.clone();
+
+    move |enr: &Enr| {
+        if let Some(bitfield_bytes) = enr.get(BITFIELD_ENR_KEY) {
+            let bitfield = match BitVector::<TSpec::SubnetBitfieldLength>::from_ssz_bytes(
+                bitfield_bytes,
+            ) {
+                Ok(v) => v,
+                Err(e) => {
+                    warn!(log_clone, "Could not decode ENR bitfield for peer"; "peer_id" => format!("{}", enr.peer_id()), "error" => format!("{:?}", e));
+                    return false;
+                }
+            };
+
+            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
+            });
+        }
+        false
+    }
+}