Separate committee subscriptions queue (#3508)

## Issue Addressed

NA

## Proposed Changes

As we've seen on Prater, there seems to be a correlation between these messages

```
WARN Not enough time for a discovery search  subnet_id: ExactSubnet { subnet_id: SubnetId(19), slot: Slot(3742336) }, service: attestation_service
```

... and nodes falling 20-30 slots behind the head for short periods. These nodes are running ~20k Prater validators.

After running some metrics, I can see that the `network_recv` channel is processing ~250k `AttestationSubscribe` messages per minute. It occurred to me that perhaps the `AttestationSubscribe` messages are "washing out" the `SendRequest` and `SendResponse` messages. In this PR I separate the `AttestationSubscribe` and `SyncCommitteeSubscribe` messages into their own queue so the `tokio::select!` in the `NetworkService` can still process the other messages in the `network_recv` channel without necessarily having to clear all the subscription messages first.

~~I've also added filter to the HTTP API to prevent duplicate subscriptions going to the network service.~~

## Additional Info

- Currently being tested on Prater

beacon_node/http_api/tests/common.rs

@@ -11,14 +11,14 @@ use lighthouse_network::{
     types::{EnrAttestationBitfield, EnrSyncCommitteeBitfield, SyncState},
     ConnectedPoint, Enr, NetworkGlobals, PeerId, PeerManager,
 };
-use network::NetworkMessage;
+use network::{NetworkReceivers, NetworkSenders};
 use sensitive_url::SensitiveUrl;
 use slog::Logger;
 use std::future::Future;
 use std::net::{IpAddr, Ipv4Addr, SocketAddr};
 use std::sync::Arc;
 use std::time::Duration;
-use tokio::sync::{mpsc, oneshot};
+use tokio::sync::oneshot;
 use types::{ChainSpec, EthSpec};
 
 pub const TCP_PORT: u16 = 42;
@@ -30,7 +30,7 @@ pub const EXTERNAL_ADDR: &str = "/ip4/0.0.0.0/tcp/9000";
 pub struct InteractiveTester<E: EthSpec> {
     pub harness: BeaconChainHarness<EphemeralHarnessType<E>>,
     pub client: BeaconNodeHttpClient,
-    pub network_rx: mpsc::UnboundedReceiver<NetworkMessage<E>>,
+    pub network_rx: NetworkReceivers<E>,
     _server_shutdown: oneshot::Sender<()>,
 }
 
@@ -41,7 +41,7 @@ pub struct ApiServer<E: EthSpec, SFut: Future<Output = ()>> {
     pub server: SFut,
     pub listening_socket: SocketAddr,
     pub shutdown_tx: oneshot::Sender<()>,
-    pub network_rx: tokio::sync::mpsc::UnboundedReceiver<NetworkMessage<E>>,
+    pub network_rx: NetworkReceivers<E>,
     pub local_enr: Enr,
     pub external_peer_id: PeerId,
 }
@@ -97,7 +97,7 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
     log: Logger,
     port: u16,
 ) -> ApiServer<T::EthSpec, impl Future<Output = ()>> {
-    let (network_tx, network_rx) = mpsc::unbounded_channel();
+    let (network_senders, network_receivers) = NetworkSenders::new();
 
     // Default metadata
     let meta_data = MetaData::V2(MetaDataV2 {
@@ -146,7 +146,7 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
             spec_fork_name: None,
         },
         chain: Some(chain.clone()),
-        network_tx: Some(network_tx),
+        network_senders: Some(network_senders),
         network_globals: Some(network_globals),
         eth1_service: Some(eth1_service),
         log,
@@ -163,7 +163,7 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
         server,
         listening_socket,
         shutdown_tx,
-        network_rx,
+        network_rx: network_receivers,
         local_enr: enr,
         external_peer_id: peer_id,
     }