Headers are a major source of slowness

Currently the metadata handling has a number of inefficiencies that materially affect performance.

Netty
- We convert MethodDescriptor into Http2Headers for every call. Could be cached
- We have no way of storing a validation result or derived value with a header cached by HPACK. E.g validating that the content-type is correct is done on the string value even though the value is effectively constant when emitted by HPACK.

Performance benchmark UnaryCallQpsBenchmark shows that header handling is ~10% of total CPU cost.