[#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/SessionCache.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections;
+
+namespace FrostFS.SDK.ClientV2;
+
+internal struct SessionCache
+{
+    public SessionCache(ulong sessionExpirationDuration)
+    {
+        TokenDuration = sessionExpirationDuration;
+    }
+
+    internal Hashtable Cache { get; } = [];
+
+    internal ulong CurrentEpoch { get; set; }
+
+    internal ulong TokenDuration { get; set; }
+
+    internal void DeleteByPrefix(string prefix)
+    {
+        foreach (var key in Cache.Keys)
+        {
+            if (((string)key).StartsWith(prefix, StringComparison.Ordinal))
+            {
+                Cache.Remove(key);
+            }
+        }
+    }
+}