Slow stream subscribers should receive a timeout

[Mirko-von-Leipzig]

#2196 adds the dropping of slow sync clients. However these can simply reconnect.

Once a client has been disconnected, we should also give them a temporary ban from subscribing. Clients on this list would be rejected from subscriptions only. The error message should also indicate the reason, and the timeout left.

There are alternatives here, but I feel like giving the slow client a e.g. 10 minute ban should suffice. Number here is arbitrary and should be larger, but it also depends on how quickly #2196 disconnects them.

My suggested approach would be to keep a basic ban: VecDequeue<(ClientIp, Instant)>. The capacity must be limited to prevent overflow. Under normal circumstances this doesn't matter, but just to be safe. We also don't need to prune the list until it reaches capacity - we just need to check the first matching Instant to see if its expired or not. This limits the complexity imo. You'll be tempted to use some sort of map instead but I don't think its worth it. ymmv, just a suggestion.

[AlgofootPrint]

I can take this.

I looked through the current block-producer path. The slow-subscriber drop already happens in SubscriptionProvider::send_event() when try_send() fails, and new streams are created from BlockProducerRpcServer::mempool_subscription().

My plan would be:

• thread the subscriber IpAddr from the gRPC request into Mempool::subscribe();

• store the active subscriber together with its IP so that a failed try_send() can register a temporary
  subscription-only ban;

• keep the ban list bounded, likely as the suggested VecDeque<(IpAddr, Instant)>;

• reject only MempoolSubscription requests while the ban is active, returning a clear tonic::Status with the
  remaining timeout;

• add focused tests for registering a ban after a slow subscriber is dropped, rejecting reconnects during the timeout,
  allowing reconnects after expiry, and keeping the ban list bounded.

  If that matches the intended direction you could assign it to me.