[#24] Client: Implement pool part1

first iteration - base classes and methods

Signed-off-by: Pavel Gross <p.gross@yadro.com>

src/FrostFS.SDK.ClientV2/Poll/InnerPool.cs

@@ -0,0 +1,47 @@
+using FrostFS.SDK.ClientV2;
+
+internal sealed class InnerPool
+{
+    private readonly object _lock = new();
+
+    internal InnerPool(Sampler sampler, ClientWrapper[] clients)
+    {
+        Clients = clients;
+        Sampler = sampler;
+    }
+
+    internal Sampler Sampler { get; set; }
+
+    internal ClientWrapper[] Clients { get; }
+
+    internal ClientWrapper? Connection()
+    {
+        lock (_lock)
+        {
+            if (Clients.Length == 1)
+            {
+                var client = Clients[0];
+                if (client.StatusMonitor.IsHealthy())
+                {
+                    return client;
+                }
+            }
+            else
+            {
+                var attempts = 3 * Clients.Length;
+
+                for (int i = 0; i < attempts; i++)
+                {
+                    int index = Sampler.Next();
+
+                    if (Clients[index].StatusMonitor.IsHealthy())
+                    {
+                        return Clients[index];
+                    }
+                }
+            }
+
+            return null;
+        }
+    }
+}