Streaming request bodies

[SeanTAllen]

Handlers currently receive the complete request body buffered in HandlerContext.body. There's no way to process body chunks incrementally as they arrive.

A streaming request body API would let handlers opt in to receive chunks as they arrive, enabling large file uploads, streaming ingestion, and other patterns where buffering the entire body is impractical.

Discussion #8 sketched an initial design for this (the "Streaming Request Bodies" section), but the details will need revisiting given how much the handler model has changed since then.