HPACK: Use optimised version of HPACK (might need to copy)

It was imported from the twitter one, however is higher performance (less allocations).
See here: https://github.com/netty/netty/tree/4.1/codec-http2/src/main/java/io/netty/handler/codec/http2/internal/hpack

To be decided if we shade or can depend on it.

More info: https://github.com/twitter/hpack/pull/32

Start by comparing the current HPACK implementation with Netty's codec-http2/src/main/java/io/netty/handler/codec/http2/internal/hpack source and review the linked Twitter HPACK pull request. Determine whether the optimized implementation should be shaded or used as a dependency; done means the chosen integration is implemented and its performance and compatibility are verified.

Written by the indexing model from the issue text.