feat: implement raft log replication and apply

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -27,6 +27,8 @@ internal sealed class RaftTestCluster
     public static RaftTestCluster Create(int nodes)
     {
         var created = Enumerable.Range(1, nodes).Select(i => new RaftNode($"n{i}")).ToList();
+        foreach (var node in created)
+            node.ConfigureCluster(created);
         return new RaftTestCluster(created);
     }
 
@@ -40,4 +42,16 @@ internal sealed class RaftTestCluster
 
         return Task.FromResult(candidate);
     }
+
+    public async Task WaitForAppliedAsync(long index)
+    {
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2));
+        while (!timeout.IsCancellationRequested)
+        {
+            if (Nodes.All(n => n.AppliedIndex >= index))
+                return;
+
+            await Task.Delay(20, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
+        }
+    }
 }