PeerManager: move the check for banned peers from connection_established (#4569)

## Issue Addressed
https://github.com/sigp/lighthouse/issues/4543
## Proposed Changes
- Removes `NotBanned` from `BanResult`, implements `Display` and `std::error::Error` for `BanResult` and changes `ban_result` return type to `Option<BanResult>` which helps returning `BanResult` on `handle_established_inbound_connection`  
- moves the check from for banned peers from `on_connection_established` to `handle_established_inbound_connection` to start addressing #4543.
- Removes `allow_block_list` as it's now redundant? Not sure about this one but if `PeerManager` keeps track of the banned peers, no need to send a `Swarm` event for `alow_block_list` to also keep that list right? 
 
## Questions

-  #4543 refers:
>  More specifically, implement the connection limit behaviour inside the peer manager.

@AgeManning do you mean copying `libp2p::connection_limits::Behaviour`'s code into `PeerManager`/ having it as an inner `NetworkBehaviour` of `PeerManager`/other? If it's the first two, I think it probably makes more sense to have it as it is as it's less code to maintain.

> Also implement the banning of peers inside the behaviour, rather than passing messages back up to the swarm.

I tried to achieve this, but we still need to pass the `PeerManagerEvent::Banned` swarm event as `DiscV5` handles it's node and ip management internally and I did not find a method to query if a peer is banned. Is there anything else we can do from here?

3397612160/beacon_node/lighthouse_network/src/discovery/mod.rs (L931-L940)

Same as the question above, I did not find a way to check if `DiscV5` has the peer banned, so that we could check here and avoid sending `Swarm` events

3397612160/beacon_node/lighthouse_network/src/peer_manager/network_behaviour.rs (L168-L178)

Is there a chance we try to dial a peer that has been banned previously? 

Thanks!

beacon_node/lighthouse_network/src/service/behaviour.rs

@@ -20,8 +20,6 @@ where
     AppReqId: ReqId,
     TSpec: EthSpec,
 {
-    /// Peers banned.
-    pub banned_peers: libp2p::allow_block_list::Behaviour<libp2p::allow_block_list::BlockedPeers>,
     /// Keep track of active and pending connections to enforce hard limits.
     pub connection_limits: libp2p::connection_limits::Behaviour,
     /// The routing pub-sub mechanism for eth2.

beacon_node/lighthouse_network/src/service/mod.rs

@@ -337,11 +337,8 @@ impl<AppReqId: ReqId, TSpec: EthSpec> Network<AppReqId, TSpec> {
             libp2p::connection_limits::Behaviour::new(limits)
         };
 
-        let banned_peers = libp2p::allow_block_list::Behaviour::default();
-
         let behaviour = {
             Behaviour {
-                banned_peers,
                 gossipsub,
                 eth2_rpc,
                 discovery,
@@ -1402,15 +1399,10 @@ impl<AppReqId: ReqId, TSpec: EthSpec> Network<AppReqId, TSpec> {
                 Some(NetworkEvent::PeerDisconnected(peer_id))
             }
             PeerManagerEvent::Banned(peer_id, associated_ips) => {
-                self.swarm.behaviour_mut().banned_peers.block_peer(peer_id);
                 self.discovery_mut().ban_peer(&peer_id, associated_ips);
                 None
             }
             PeerManagerEvent::UnBanned(peer_id, associated_ips) => {
-                self.swarm
-                    .behaviour_mut()
-                    .banned_peers
-                    .unblock_peer(peer_id);
                 self.discovery_mut().unban_peer(&peer_id, associated_ips);
                 None
             }
@@ -1459,7 +1451,6 @@ impl<AppReqId: ReqId, TSpec: EthSpec> Network<AppReqId, TSpec> {
             let maybe_event = match swarm_event {
                 SwarmEvent::Behaviour(behaviour_event) => match behaviour_event {
                     // Handle sub-behaviour events.
-                    BehaviourEvent::BannedPeers(void) => void::unreachable(void),
                     BehaviourEvent::Gossipsub(ge) => self.inject_gs_event(ge),
                     BehaviourEvent::Eth2Rpc(re) => self.inject_rpc_event(re),
                     // Inform the peer manager about discovered peers.