Made the RandomHelper thread-safe. (#542)

* Made the RandomHelper thread-safe.

* Made the RandomHelper thread-safe.

src/EasyCaching.Core/Internal/RandomHelper.cs

@@ -1,20 +1,30 @@
 namespace EasyCaching.Core.Internal
 {
     using System;
+    using System.Threading;
 
     public static class RandomHelper
     {
-#if NETSTANDARD2_0
-        private static readonly Random _random = new Random();
+#if NET6_0_OR_GREATER
+        private static Random Instance => Random.Shared;
+#else
+        private static Random Instance => _local.Value;
+
+        private static readonly Random _global = new Random();
+        private static readonly ThreadLocal<Random> _local = new ThreadLocal<Random>(() =>
+        {
+            int seed;
+            lock (_global)
+            {
+                seed = _global.Next();
+            }
+            return new Random(seed);
+        });
 #endif
 
         public static int GetNext(int min, int max)
         {
-#if NET6_0_OR_GREATER
-            return Random.Shared.Next(min, max);
-#else
-            return _random.Next(min, max);
-#endif
+            return Instance.Next(min, max);
         }
     }
 }