Maintain peers across all data column subnets (sigp#7915)

Closes:
- sigp#7865
- sigp#7855

Changes extracted from earlier PR sigp#7876

This PR fixes two main things with a few other improvements mentioned below:
- Prevent Lighthouse from repeatedly sending `DataColumnByRoot` requests to an unsynced peer, causing lookup sync to get stuck
- Allows Lighthouse to send discovery requests if there isn't enough **synced** peers in the required sampling subnets - this fixes the stuck sync scenario where there isn't enough usable peers in sampling subnet but no discovery is attempted.


  - Make peer discovery queries if custody subnet peer count drops below the minimum threshold
- Update peer pruning logic to prioritise uniform distribution across all data column subnets and avoid pruning sampling peers if the count is below the target threshold (2)
- Check sync status when making discovery requests, to make sure we don't ignore requests if there isn't enough synced peers in the required sampling subnets
- Optimise some of the `PeerDB` functions checking custody peers
- Only send lookup requests to peers that are synced or advanced

Author: jimmygchen

beacon_node/lighthouse_network/src/discovery/mod.rs

@@ -1223,7 +1223,7 @@ impl<E: EthSpec> Discovery<E> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::rpc::methods::{MetaData, MetaDataV2};
+    use crate::rpc::methods::{MetaData, MetaDataV3};
     use libp2p::identity::secp256k1;
     use types::{BitVector, MinimalEthSpec, SubnetId};
 
@@ -1248,10 +1248,11 @@ mod tests {
         .unwrap();
         let globals = NetworkGlobals::new(
             enr,
-            MetaData::V2(MetaDataV2 {
+            MetaData::V3(MetaDataV3 {
                 seq_number: 0,
                 attnets: Default::default(),
                 syncnets: Default::default(),
+                custody_group_count: spec.custody_requirement,
             }),
             vec![],
             false,