Maintain peers across all data column subnets (#7915)

Closes:
- #7865
- #7855

Changes extracted from earlier PR #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

beacon_node/network/src/sync/backfill_sync/mod.rs

@@ -1120,13 +1120,12 @@ impl<T: BeaconChainTypes> BackFillSync<T> {
                 .sampling_subnets()
                 .iter()
                 .all(|subnet_id| {
-                    let peer_count = network
+                    let min_peer_count = 1;
+                    network
                         .network_globals()
                         .peers
                         .read()
-                        .good_range_sync_custody_subnet_peers(*subnet_id)
-                        .count();
-                    peer_count > 0
+                        .has_good_peers_in_custody_subnet(subnet_id, min_peer_count)
                 })
         } else {
             true