HPACK decoder fixes (dotnet/corefx#39907)

HPACK decoder fixes (dotnet/corefx#39907)

- Fix HPACK decoder failing inconsistently with 0-length header names. Fail fast with same exception message that HTTP1 uses. Resolves dotnet/corefx#39873.
- Fix `DynamicTable` returning incorrect values or throwing exceptions when it wraps its ring buffer. Resolves dotnet/corefx#39656.
- Moves HPACK encoder methods from `Http2LoopbackConnection` into their own class.
- Implement all forms of header encodings in HPACK encoder.
- Move a number of files where they belong into the ProductionCode folder in unit test project.

Commit migrated from https://github.com/dotnet/corefx/commit/b7e60c9aa17fbe043318afd68af31cd3a6d61a76