Use Monitor.TryEnter/Exit in the Scan loop (#189)

- Avoids overlapping Scans
- Noops if dispose is being called at the same time

src/Microsoft.AspNetCore.Sockets/ConnectionManager.cs

@@ -87,7 +87,16 @@ namespace Microsoft.AspNetCore.Sockets
 
         private void Scan()
         {
-            lock (_executionLock)
+            // If we couldn't get the lock it means one of 2 things is true:
+            // - We're about to dispose so we don't care to run the scan callback anyways.
+            // - The previous Scan took long enough that the next scan tried to run in parallel
+            // In either case just do nothing and end the timer callback as soon as possible
+            if (!Monitor.TryEnter(_executionLock))
+            {
+                return;
+            }
+
+            try
             {
                 if (_disposed)
                 {
@@ -97,8 +106,6 @@ namespace Microsoft.AspNetCore.Sockets
                 // Pause the timer while we're running
                 _timer.Change(Timeout.Infinite, Timeout.Infinite);
 
-                try
-                {
                 // Scan the registered connections looking for ones that have timed out
                 foreach (var c in _connections)
                 {
@@ -130,7 +137,9 @@ namespace Microsoft.AspNetCore.Sockets
             {
                 // Resume once we finished processing all connections
                 _timer.Change(TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(1));
-                }
+
+                // Exit the lock now
+                Monitor.Exit(_executionLock);
             }
         }