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

common/eth2/src/types.rs

@@ -682,7 +682,7 @@ pub struct ValidatorAggregateAttestationQuery {
     pub slot: Slot,
 }
 
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Hash)]
 pub struct BeaconCommitteeSubscription {
     #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub validator_index: u64,