Remove race condition from socket input that could stall reads

- Without the _sync lock, if new data was produced as ConsumingComplete
  was called, the next "await SocketInput" might never complete despite not
  all data being examined.
- If more data is produced afterward, the stall would be prevented, but this
  isn't always the case such as during the end of the request.

src/Microsoft.AspNetCore.Server.Kestrel/Http/SocketInput.cs

@@ -28,6 +28,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         private MemoryPoolBlock _pinned;
 
         private int _consumingState;
+        private object _sync = new object();
 
         public SocketInput(MemoryPool memory, IThreadPool threadPool)
         {
@@ -57,6 +58,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         }
 
         public void IncomingData(byte[] buffer, int offset, int count)
+        {
+            lock (_sync)
             {
                 if (count > 0)
                 {
@@ -82,8 +85,11 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
 
                 Complete();
             }
+        }
 
         public void IncomingComplete(int count, Exception error)
+        {
+            lock (_sync)
             {
                 if (_pinned != null)
                 {
@@ -117,6 +123,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
 
                 Complete();
             }
+        }
 
         public void IncomingDeferred()
         {
@@ -161,6 +168,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         public void ConsumingComplete(
             MemoryPoolIterator consumed,
             MemoryPoolIterator examined)
+        {
+            lock (_sync)
             {
                 MemoryPoolBlock returnStart = null;
                 MemoryPoolBlock returnEnd = null;
@@ -198,6 +207,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
                     throw new InvalidOperationException("No ongoing consuming operation to complete.");
                 }
             }
+        }
 
         public void CompleteAwaiting()
         {