perf: remaining small items

[mattklein123]

A few more small items for this perf pass before calling it good:

• Don't allocate PooledStreamEncoder (class can be merged into Router, not used anywhere else)
• Cache date string for date header every 500ms or so
• Store integer headers as integers also to avoid converting back and forth
• Optimize buffer callbacks
• Remove locking from libevent base
• 0 copy grpc encode
• Only do PNRG in runtime when needed
• stat flushing code
• Don't copy http/2 static headers when encoding
• remove fmt::format from dynamic stats in router
• remove copy from path match route action
• allocate "light" host when doing service discovery so we don't allocate full stat block which we never use.
• std::vector<nghttp2_settings_entry> iv in http2 codec can be a fixed array

[mattklein123]

#220

[mattklein123]

#221

[mattklein123]

#244

[mattklein123]

I'm closing this out. We can open individual issues for perf things that come up on traces.