Additional streaming APIs

[aantron]

• There is already file upload string streaming exposed. Dream may need to expose bigstring streaming for zero-copy uploads.
• WebSocket frame-by-frame chunk-by-chunk streaming of large messages. Probably only a bigstring API for this. Done in Implement WebSockets on top of the new streams #178, though not fully exposed in the API yet.
  • Streaming out messages is currently not possible, because frames are hardcoded with is_fin:true upstream.
• Stream limits and handling in the GraphQL helpers.
• Upload stream error handling currently raises exception on bad Content-Type.
• iovec conversion.
• Stream static files.