OnManagedConnectionFailed thread/memory leak fix (#1710)

Hi. I faced a problem with 'ConnectionMultiplexer': when it loses connection with Redis cluster, 'OnManagedConnectionFailed' event handler creates timer, which tries to 'SwitchMaster'. Timer ticks every second, but there is PLINQ query in 'GetConfiguredMasterForService' method, that leads to constantly threads starting (threads start faster than they exit). And if disconnect time is long enough, I get thread/memory leak in service.
PR contains hotfix of a problem with timer (timer starts next iteration only after previous iteration complete). But maybe you should take a closer look at PLINQ queries (they seem superfluous).
Also, I commented `TextWriter.Null` creation in 'SwitchMaster', because it seems like useless memory allocations (you handle nullable LogProxy).

Author: alexSatov

src/StackExchange.Redis/ConnectionMultiplexer.cs

@@ -2477,11 +2477,9 @@ internal void OnManagedConnectionRestored(object sender, ConnectionFailedEventAr
         {
             ConnectionMultiplexer connection = (ConnectionMultiplexer)sender;
 
-            if (connection.sentinelMasterReconnectTimer != null)
-            {
-                connection.sentinelMasterReconnectTimer.Dispose();
-                connection.sentinelMasterReconnectTimer = null;
-            }
+            var oldTimer = Interlocked.Exchange(ref connection.sentinelMasterReconnectTimer, null);
+            oldTimer?.Dispose();
+
             try
             {
 
@@ -2524,7 +2522,7 @@ internal void OnManagedConnectionFailed(object sender, ConnectionFailedEventArgs
             // or if we miss the published master change
             if (connection.sentinelMasterReconnectTimer == null)
             {
-                connection.sentinelMasterReconnectTimer = new Timer((_) =>
+                connection.sentinelMasterReconnectTimer = new Timer(_ =>
                 {
                     try
                     {
@@ -2533,9 +2531,12 @@ internal void OnManagedConnectionFailed(object sender, ConnectionFailedEventArgs
                     }
                     catch (Exception)
                     {
-
                     }
-                }, null, TimeSpan.FromSeconds(0), TimeSpan.FromSeconds(1));
+                    finally
+                    {
+                        connection.sentinelMasterReconnectTimer?.Change(TimeSpan.FromSeconds(1), Timeout.InfiniteTimeSpan);
+                    }
+                }, null, TimeSpan.Zero, Timeout.InfiniteTimeSpan);
             }
         }
 
@@ -2730,6 +2731,8 @@ public void Dispose()
             GC.SuppressFinalize(this);
             Close(!_isDisposed);
             sentinelConnection?.Dispose();
+            var oldTimer = Interlocked.Exchange(ref sentinelMasterReconnectTimer, null);
+            oldTimer?.Dispose();
         }
 
         internal Task<T> ExecuteAsyncImpl<T>(Message message, ResultProcessor<T> processor, object state, ServerEndPoint server)