Use the IThreadPool as the ReaderScheduler (#1372)

- This is what socket input did, also using
ThreadPool.QueueUserWorkItem is better than Task.Run for this
scenario and avoids a bunch of overhead.

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/ListenerContext.cs

@@ -44,7 +44,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
 
         public PipeOptions LibuvPipeOptions => new PipeOptions
         {
-            ReaderScheduler = TaskRunScheduler.Default,
+            ReaderScheduler = ServiceContext.ThreadPool,
             WriterScheduler = Thread,
             MaximumSizeHigh = ServiceContext.ServerOptions.Limits.MaxRequestBufferSize ?? 0,
             MaximumSizeLow = ServiceContext.ServerOptions.Limits.MaxRequestBufferSize ?? 0

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Infrastructure/IThreadPool.cs

@@ -2,12 +2,13 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.IO.Pipelines;
 using System.Threading;
 using System.Threading.Tasks;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
 {
-    public interface IThreadPool
+    public interface IThreadPool : IScheduler
     {
         void Complete(TaskCompletionSource<object> tcs);
         void Cancel(TaskCompletionSource<object> tcs);

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Infrastructure/LoggingThreadPool.cs

@@ -112,5 +112,10 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
                 }
             }, tcs);
         }
+
+        public void Schedule(Action action)
+        {
+            Run(action);
+        }
     }
 }

test/Microsoft.AspNetCore.Server.KestrelTests/TestHelpers/SynchronousThreadPool.cs

@@ -34,5 +34,10 @@ namespace Microsoft.AspNetCore.Server.KestrelTests.TestHelpers
         {
             action(state);
         }
+
+        public void Schedule(Action action)
+        {
+            action();
+        }
     }
 }