runtime: HTTP2: Flakyness when server doesn't completely read request

I have created a repro for an error that sometimes shows up on the grpc-dotnet CI server.

Repro:

1. git clone https://github.com/JamesNK/grpc-dotnet.git
2. git checkout jamesnk/stress-maxsize
3. dotnet test test\FunctionalTests --filter Name~ReceivedMessageExceedsSize_ThrowError

Logic in the test is like:

1. Client makes a non-streaming call to the server and sends a large request body
2. Server ends response without reading complete request body
3. Client asserts response
4. Go to 1.

When one thread is executing this will not error.

When multiple threads share one HttpClient it consistently throws this error:

System.Threading.Tasks.TaskCanceledException : A task was canceled.
  Stack Trace: 
    at TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
    at Http2Connection.SendStreamDataAsync(Int32 streamId, ReadOnlyMemory`1 buffer, CancellationToken cancellationToken)
    at Http2Stream.SendDataAsync(ReadOnlyMemory`1 buffer, CancellationToken cancellationToken)
    at HttpContent.CopyToAsyncCore(ValueTask copyTask)
    at Http2Stream.SendRequestBodyAsync(CancellationToken cancellationToken)
    at Http2Connection.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    at HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
    at RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    at HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
    at TaskExtensions.TimeoutAfter[T](Task`1 task, TimeSpan timeout, String filePath, Int32 lineNumber) in TaskExtensions.cs line: 58
    at <<ReceivedMessageExceedsSize_ThrowError>b__1>d.MoveNext() in MaxMessageSizeTests.cs line: 77
    at --- End of stack trace from previous location where exception was thrown ---
    at MaxMessageSizeTests.ReceivedMessageExceedsSize_ThrowError() in MaxMessageSizeTests.cs line: 87
    at GenericAdapter`1.BlockUntilCompleted()
    at NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter awaitable)
    at AsyncToSyncAdapter.Await(Func`1 invoke)
    at TestMethodCommand.RunTestMethod(TestExecutionContext context)
    at TestMethodCommand.Execute(TestExecutionContext context)
    at <>c__DisplayClass1_0.<Execute>b__0()
    at BeforeAndAfterTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)

.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview9-013697
 Commit:    baac5f4d5c

// @geoffkizer