System.Buffers + Json.NET everywhere

[JamesNK]

The JSON input and output formatters use array pooling from System.Buffers.

Other places to add System.Buffers:

• JsonViewComponentResult -

  Mvc/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/JsonViewComponentResult.cs

  Line 75 in 25eb501

  jsonWriter.CloseOutput = false;

• JsonResultExecutor -

  Mvc/src/Microsoft.AspNetCore.Mvc.Formatters.Json/Internal/JsonResultExecutor.cs

  Line 114 in 8b1bd34

  jsonWriter.CloseOutput = false;

[Eilon]

Will discuss tomorrow with a few folks. Sorta kinda related to the mess described in #3992.

[Eilon]

Discussed with @dougbu and @rynowak (The Outlaw):

In #3992 we decided to remove JsonViewComponentResult, so that's not an issue here.

For JsonResultExecutor we took a look at the code and it already uses pooling by virtue of IHttpResponseStreamWriterFactory, which in its implementation uses the pooling feature.

[JamesNK]

Json.NET doesn't automatically use pooling for its internal buffer because of IHttpResponseStreamWriterFactory.

From JsonOutputFormatter:

Mvc/src/Microsoft.AspNetCore.Mvc.Formatters.Json/JsonOutputFormatter.cs

Line 120 in 4123d83

ArrayPool = _charPool,

[rynowak]

Ah ok yeah, you're right. We should do this.