Go HTTP/2 performance

[emcfarlane]

Benchmarking shows gRPC methods around 1.5x slower then HTTP/1 methods. Meta issue to track other issues:

• x/net/http2: support memory re-use for MetaHeadersFrames golang/go#22717
• net/http: http/2 throughput is very slow compared to http/1.1 golang/go#47840
• net/http: Stream starvation in http2 priority write scheduler golang/go#58804