Refactor attestation service (#1415)

## Issue Addressed

N/A

## Proposed Changes

Refactor attestation service to send out requests to find peers for subnets as soon as we get attestation duties. 
Earlier, we had much more involved logic to send the discovery requests to the discovery service only 6 slots before the attestation slot. Now that discovery is much smarter with grouped queries, the complexity in attestation service can be reduced considerably.



Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -1,6 +1,6 @@
 use crate::peer_manager::{score::PeerAction, PeerManager, PeerManagerEvent};
 use crate::rpc::*;
-use crate::types::{EnrBitfield, GossipEncoding, GossipKind, GossipTopic};
+use crate::types::{EnrBitfield, GossipEncoding, GossipKind, GossipTopic, SubnetDiscovery};
 use crate::Eth2Enr;
 use crate::{error, metrics, Enr, NetworkConfig, NetworkGlobals, PubsubMessage, TopicHash};
 use futures::prelude::*;
@@ -29,7 +29,6 @@ use std::{
     marker::PhantomData,
     sync::Arc,
     task::{Context, Poll},
-    time::Instant,
 };
 use types::{EnrForkId, EthSpec, SignedBeaconBlock, SubnetId};
 
@@ -301,8 +300,9 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
 
     /// Attempts to discover new peers for a given subnet. The `min_ttl` gives the time at which we
     /// would like to retain the peers for.
-    pub fn discover_subnet_peers(&mut self, subnet_id: SubnetId, min_ttl: Option<Instant>) {
-        self.peer_manager.discover_subnet_peers(subnet_id, min_ttl)
+    pub fn discover_subnet_peers(&mut self, subnet_subscriptions: Vec<SubnetDiscovery>) {
+        self.peer_manager
+            .discover_subnet_peers(subnet_subscriptions)
     }
 
     /// Updates the local ENR's "eth2" field with the latest EnrForkId.