Refactor request body streaming state handling

[tanner0101]

• Implements new state machine for handling streaming request bodies (Refactor request body streaming state handling #2357).

This new state machine ensures that calls to req.body.drain will only ever happen after the previously returned future has completed. This makes it easier to correctly implement streaming file writes. Addresses https://forums.swift.org/t/how-to-use-nonblockingfileio-for-repeated-writes/36206.

• Request bodies are now automatically drained after sending a response (Refactor request body streaming state handling #2357).

This change ensures that streaming requests will be read completely even if a route ignores their body.

• Adds a streaming file upload example to the Development executable (Refactor request body streaming state handling #2357).

• Improves BodyStreamResult's normal and debug descriptions (Refactor request body streaming state handling #2357).

• Fixed a reference cycle if Request was captured strongly within the body.drain closure (Refactor request body streaming state handling #2357).

[mcritz]

@tanner0101 Thanks for working on this. Much appreciated.

I assume this will address concerns with https://forums.swift.org/t/how-to-use-nonblockingfileio-for-repeated-writes/36206/16 ?

[tanner0101]

Yup. That's what triggered this. I just need to write tests to verify the new state machine and then this is good to go.

[tanner0101]

These changes are now available in 4.7.0

[mcritz]

Many thanks!