Convert Write100ContinueAsync to sync

[Tratcher]

IHttpOutputProducer.Write100ContinueAsync is only ever called synchronously from HttpProtocol.ProduceContinue. It can and should be reimplemented synchronously.

https://github.com/aspnet/KestrelHttpServer/blob/2191327b59f87f23f69fff2ac0dba9e58b67141b/src/Kestrel.Core/Internal/Http/HttpProtocol.cs#L969

[analogrelay]

Way too marginal :)