feat: Peer management with shard as a dimension

[chair28980]

If we assume that peer discovery takes care of filtering peers by shard, these peers must now be managed in a way that makes sense in a dynamic auto-sharded environment. Some requirements/ideas:

• peer manager must preferably try to keep at least a target n peers for each subscribed shard (note that dynamically subscribing/unsubscribing from shards could complicate this)
• peer manager must attempt to maintain a healthy relay connectivity for each subscribed shard
• a mechanism to perform ad-hoc discovery (or another solution) to cater for when a node subscribes to a shard for which there are no tracked peers

Priority: Critical for launch

[chaitanyaprem]

Closing this issue as first two items have already been addressed via #727. It applies for both static and autosharding.
The 3rd item, i.e ad-hoc discovery is being tracked via #680.