SelfHost: Change to callback based responses for better performance

[jocull]

Prerequisites

• [X] I have written a descriptive pull-request title
• [X] I have verified that there are no overlapping pull-requests open
• [X] I have verified that I am following the Nancy code style guidelines
• [X] I have provided test coverage for my change (where applicable)

Description

I noticed that the SelfHost was not performing well when using async methods. You can test a simple endpoint using await Task.Delay(100) and see them queue up as the engine handle blocks waiting for the task. This change converts it to complete/error callbacks so they no longer block. Hope it helps, and please let me know if I missed anything! I'm new here :)

[dnfclas]

All CLA requirements met.

[thecodejunkie]

@khellang this is a PR sent tot he 1.x-WorkingBranch @jocull 1.x has been in maintenance mode for quite some time now and I'd rather see you sent this PR against master

[khellang]

But that overload isn't there in 2.0. We've moved everything over to async/await and Tasks.

[jocull]

I can try to check this out in 2.x - it may not be an issue in the new versions, but was an issue in the current stable.

[thecodejunkie]

@jocull yeah it would be great if you could check if it happens on master / latest nuget release

[jocull]

@thecodejunkie Alright, so here's what I turned up:

• The latest revision master is all good, it benchmarks just fine
• The latest on pre-release on nuget 2.0.0-clinteastwood has the same issue as 1.4.4 (didn't investigate, it just benchmarks that way)
• And of course, the latest stable on nuget 1.4.4 also has this problem, which is patched in this PR

Along the way I noticed that 2.0.0-clinteastwood is very old, over a year, but no new pre-releases to nuget. Is there a roadmap somewhere that shows the 2.x plans?