perf: more efficient header processing

[mattklein123]

As part of recent profiling comparison with nginx:

Right now how Envoy handles headers is extremely inefficient. We use a list, along with std::string, and generally iterate over the entire list for remove operations, etc.

Move to a more efficient implementation that:

• Looks at each header on receive and store direct pointer into the list if Envoy will ever use it (via hash lookup). All add/remove/modify operations become O(1).
• Move away from std::string, implement explicit short string optimization, with sentinel for common headers.

[mattklein123]

#209

[mattklein123]

fixed