Merge http2 grpc fixes

[pcapriotti]

We now depend on a fork of http2-grpc-haskell containing fixes for:

haskell-grpc-native/http2-grpc-haskell#46
haskell-grpc-native/http2-grpc-haskell#48

as well as the master of http2-client:

https://github.com/lucasdicioccio/http2-client

Checklist

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