Add FlushAllAsync api

Enyim.Caching/IMemcachedClient.cs

@@ -66,6 +66,7 @@ public interface IMemcachedClient : IDisposable
         Task<bool> RemoveAsync(string key);
 
         void FlushAll();
+        Task FlushAllAsync();
 
         ServerStats Stats();
         ServerStats Stats(string type);

Enyim.Caching/MemcachedClient.cs

@@ -884,6 +884,20 @@ public void FlushAll()
             }
         }
 
+        public async Task FlushAllAsync()
+        {
+            var tasks = new List<Task>();
+
+            foreach (var node in this.pool.GetWorkingNodes())
+            {
+                var command = this.pool.OperationFactory.Flush();
+
+                tasks.Add(node.ExecuteAsync(command));
+            }
+
+            await Task.WhenAll(tasks);
+        }
+
         /// <summary>
         /// Returns statistics about the servers.
         /// </summary>

Enyim.Caching/NullMemcachedClient.cs

@@ -252,5 +252,10 @@ public Task<T> GetValueOrCreateAsync<T>(string key, int cacheSeconds, Task<T> ge
         {
             return generator;
         }
+
+        public Task FlushAllAsync()
+        {
+            return Task.CompletedTask;
+        }
     }
 }

MemcachedTest/MemcachedClientTest.cs

@@ -358,6 +358,27 @@ public void IncrementLongTest()
                 Assert.Equal(initialValue + 24, client.Increment("VALUE", 10UL, 24UL));
             }
         }
+
+        [Fact]
+        public async Task FlushTest()
+        {
+            using (MemcachedClient client = GetClient())
+            {
+                for (int i = 0; i < 10; i++)
+                {
+                    string cacheKey = $"Hello_Flush_{i}";
+                    Assert.True(await client.StoreAsync(StoreMode.Set, cacheKey, i, DateTime.Now.AddSeconds(30)));
+                }
+
+                await client.FlushAllAsync();
+
+                for (int i = 0; i < 10; i++)
+                {
+                    string cacheKey = $"Hello_Flush_{i}";
+                    Assert.Null(await client.GetValueAsync<string>(cacheKey));
+                }
+            }
+        }
     }
 }