[Testing] Combined Alloc PRs for testing

[benaadams]

PRs
#424 Improved RequestProcessingAsync legibility
#411 [Proposal] Request header Stringpool
#408 Remove CreateLinkedTokenSource
#363 Resuse writes, initalize queues

Before 7,625,384 bytes used for 4k requests

After 3,817,438 bytes used for 4k requests

Down 3,807,946 bytes (~50% reduction)

[benaadams]

Remaining big spenders (4k requests):

Function Name	Exclusive Bytes
Microsoft.AspNet.Hosting.Internal.HostingEngine.<>.<>d.MoveNext()	160000
Microsoft.AspNet.Hosting.Internal.RequestServicesContainerMiddleware..MoveNext()	192000
Microsoft.AspNet.Http.Internal.DefaultHttpContext..ctor()	512000
Microsoft.AspNet.Http.Internal.HttpContextFactory.Create()	448000
Microsoft.AspNet.Server.Kestrel.Http.Frame.d__104.MoveNext()	640000
Microsoft.AspNet.Server.Kestrel.Http.MessageBody.For()	160000
Microsoft.AspNet.Server.Kestrel.Http.SocketOutput.WriteAsync()	288
Microsoft.AspNet.Server.Kestrel.Http.SocketOutput.WriteContext..ctor()	408
Microsoft.AspNet.Server.Kestrel.KestrelThread.DoPostWork()	192000
System.Collections.Concurrent.ConcurrentQueue`1.Enqueue()	8800
System.Collections.Generic.List`1..ctor()	192
System.DateTimeFormat.Format amortized by time not request	11200
System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted	844080
System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted	338312
System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetResult(!0)	704
System.String.CtorCharPtrStartLength	133926
System.Threading.ThreadPool.QueueUserWorkItem	175528

[AspNetSmurfLab]

Very nice gain of ~9%:

Running 10s test @ http://10.0.0.100:5001/plaintext
  32 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    19.22ms   52.34ms 537.25ms   89.63%
    Req/Sec    23.94k     2.43k   38.72k    74.73%
  7616398 requests in 10.10s, 0.94GB read
  Socket errors: connect 0, read 0, write 299, timeout 0
Requests/sec: 754102.55
Transfer/sec:     94.93MB

[benaadams]

Put [Proposal] back on #411 "Request header Stringpool" needs to looked at in isolation since there wasn't a giant leap.

[ErikSchierboom]

A 50% reduction in memory, the GC will like that 👍

[benaadams]

@halter73 need to do some updates to this.