Investigate performance of large writes on Kestrel

When testing writing a 1MB chunked response, we spend a considerable amount of time copying buffers (this profile is on windows):


The above shows:
- 20% of the time copying into kernel buffers
- 10% of the time copying data from the user's buffer into Kestrel's buffer using 4K blocks
- 2% of the time in GetSpan

Here's what the flow looks like for HTTP/1.1 connections:

## HTTPS

- Socket.SendAsync(...) -> copies buffers into kernel
- PipeWriter.WriteAsync(...) -> copies buffer then flushes
- SslStream.WriteAsync(...) -> copies and encrypts individual buffers
- StreamPipeWriter.FlushAsync() ~copies buffer then flushes~ (copy removed in .NET 6)
- ConcurrentPipeWriter.WriteAsync() (usually passthrough)
- Http1OutputProducer.WriteAsync(buffer)
- HttpResponse.Body.WriteAsync(buffer)

## HTTP

- Socket.SendAsync(...) -> copies buffers into kernel buffer
- PipeWriter.WriteAsync(...) -> copies buffer then flushes
- ConcurrentPipeWriter.WriteAsync() (usually passthrough)
- Http1OutputProducer.WriteAsync(buffer)
- HttpResponse.Body.WriteAsync(buffer)

Packet sizes on the wire look good for both TLS and non-TLS connections:

Non-TLS


TLS


Code sample:

```C#
using System;
using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

var host = new HostBuilder().ConfigureWebHostDefaults(host =>
{
host.Configure(app =>
{
var s = Encoding.UTF8.GetBytes(new string('A', 1024 * 512)).AsMemory();

async Task Hello(HttpContext context)
{
await context.Response.StartAsync();

var source = s;

var memory = context.Response.BodyWriter.GetMemory();

source[..memory.Length].CopyTo(memory);

context.Response.BodyWriter.Advance(memory.Length);

source = source[memory.Length..];

// context.Response.ContentLength = s.Length;
await context.Response.Body.WriteAsync(source);
}

app.UseRouting();

app.UseEndpoints(routes =>
{
routes.MapGet("/", Hello);
});
});
}).Build();

await host.RunAsync();
```