Use Huffman code in HPackEncoder

[JunTaoLuo]

Currently we are using a direct encoding when we encode headers. We can optimize by using Huffman code: https://tools.ietf.org/html/rfc7541#section-2.4.

Related work:

• Static compression dotnet core web api exception handling - unhandled exceptions #2950
• Dynamic compression OneDrive on the roadmap? #2951

[JunTaoLuo]

Might be able to leverage the code at: https://github.com/aspnet/KestrelHttpServer/blob/423d5432be5206849cd90cc0f524aed0ec8d028b/src/Kestrel.Core/Internal/Http2/HPack/Huffman.cs.

[scalablecory]

See dotnet/corefx#41805 where I updated our shared HPackEncoder to support this.

I prototyped it a few different ways and found that even with lots of optimization, the CPU usage introduced by Huffman coding was unacceptable.