Improved peer management (#2993)

## Issue Addressed

I noticed in some logs some excess and unecessary discovery queries. What was happening was we were pruning our peers down to our outbound target and having some disconnect. When we are below this threshold we try to find more peers (even if we are at our peer limit). The request becomes futile because we have no more peer slots. 

This PR corrects this issue and advances the pruning mechanism to favour subnet peers. 

An overview the new logic added is:
- We prune peers down to a target outbound peer count which is higher than the minimum outbound peer count.
- We only search for more peers if there is room to do so, and we are below the minimum outbound peer count not the target. So this gives us some buffer for peers to disconnect. The buffer is currently 10%

The modified pruning logic is documented in the code but for reference it should do the following:
- Prune peers with bad scores first
- If we need to prune more peers, then prune peers that are subscribed to a long-lived subnet
- If we still need to prune peers, the prune peers that we have a higher density of on any given subnet which should drive for uniform peers across all subnets.

This will need a bit of testing as it modifies some significant peer management behaviours in lighthouse.

beacon_node/network/src/subnet_service/attestation_subnets.rs

@@ -23,7 +23,7 @@ use crate::metrics;
 
 /// The minimum number of slots ahead that we attempt to discover peers for a subscription. If the
 /// slot is less than this number, skip the peer discovery process.
-/// Subnet discovery query takes atmost 30 secs, 2 slots take 24s.
+/// Subnet discovery query takes at most 30 secs, 2 slots take 24s.
 const MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD: u64 = 2;
 /// The time (in slots) before a last seen validator is considered absent and we unsubscribe from the random
 /// gossip topics that we subscribed to due to the validator connection.

beacon_node/network/src/subnet_service/sync_subnets.rs

@@ -21,7 +21,7 @@ use crate::metrics;
 
 /// The minimum number of slots ahead that we attempt to discover peers for a subscription. If the
 /// slot is less than this number, skip the peer discovery process.
-/// Subnet discovery query takes atmost 30 secs, 2 slots take 24s.
+/// Subnet discovery query takes at most 30 secs, 2 slots take 24s.
 const MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD: u64 = 2;
 
 /// A particular subnet at a given slot.
@@ -115,7 +115,7 @@ impl<T: BeaconChainTypes> SyncCommitteeService<T> {
             metrics::inc_counter(&metrics::SYNC_COMMITTEE_SUBSCRIPTION_REQUESTS);
             //NOTE: We assume all subscriptions have been verified before reaching this service
 
-            // Registers the validator with the subnet  service.
+            // Registers the validator with the subnet service.
             // This will subscribe to long-lived random subnets if required.
             trace!(self.log,
                 "Sync committee subscription";