Shift subnet backbone structure (attnets revamp) (#4304)

This PR address the following spec change: https://github.com/ethereum/consensus-specs/pull/3312

Instead of subscribing to a long-lived subnet for every attached validator to a beacon node, all beacon nodes will subscribe to `SUBNETS_PER_NODE` long-lived subnets. This is currently set to 2 for mainnet. 

This PR does not include any scoring or advanced discovery mechanisms. A future PR will improve discovery and we can implement scoring after the next hard fork when we expect all client teams and all implementations to respect this spec change.

This will be a significant change in the subnet network structure for consensus clients and we will likely have to monitor and tweak our peer management logic.

beacon_node/network/src/service.rs

@@ -317,8 +317,7 @@ impl<T: BeaconChainTypes> NetworkService<T> {
         // attestation subnet service
         let attestation_service = AttestationService::new(
             beacon_chain.clone(),
-            #[cfg(feature = "deterministic_long_lived_attnets")]
-            network_globals.local_enr().node_id().raw().into(),
+            network_globals.local_enr().node_id(),
             config,
             &network_log,
         );