synchronize access to native channel methods

src/csharp/Grpc.Core/Channel.cs

@@ -152,8 +152,11 @@ namespace Grpc.Core
                 "Shutdown is a terminal state. No further state changes can occur.");
             var tcs = new TaskCompletionSource<object>();
             var deadlineTimespec = deadline.HasValue ? Timespec.FromDateTime(deadline.Value) : Timespec.InfFuture;
+            lock (myLock)
+            {
                 // pass "tcs" as "state" for WatchConnectivityStateHandler.
                 handle.WatchConnectivityState(lastObservedState, deadlineTimespec, completionQueue, WatchConnectivityStateHandler, tcs);
+            }
             return tcs.Task;
         }
 
@@ -236,7 +239,10 @@ namespace Grpc.Core
                 Logger.Warning("Channel shutdown was called but there are still {0} active calls for that channel.", activeCallCount);
             }
 
+            lock (myLock)
+            {
                 handle.Dispose();
+            }
 
             await Task.WhenAll(GrpcEnvironment.ReleaseAsync(), connectivityWatcherTask).ConfigureAwait(false);
         }
@@ -284,9 +290,12 @@ namespace Grpc.Core
         private ChannelState GetConnectivityState(bool tryToConnect)
         {
             try
+            {
+                lock (myLock)
                 {
                     return handle.CheckConnectivityState(tryToConnect);
                 }
+            }
             catch (ObjectDisposedException)
             {
                 return ChannelState.Shutdown;