Add write_timeout option

[jordanlibrande]

This adds a write_timeout option, similar to existing option read_timeout, but for timeout while writing responses.

The usecase is to detect and fail early if a client doesn't promptly read & ack response packets. (Either regular client, or intentionally malicious client doing a "slow read" attack).

In order to implement it properly and have identical semantics to read_timeout, I needed to refactor to switch sending to be chunk-based.

I'm new to this codebase, feedback would be great!

The other things we could do:

• just be ok with write_timeout not being chunk-based the same way read_timeout is, and have it be for the entire response (though I'm worried this wouldn't be as reliable detection of slow clients)
• make MAX_SEND configurable as well?
• if write_timeout not set, use the existing code, otherwise if write_timeout is set we do the chunk-based.

[jordanlibrande]

I was able to work around this I think satisfactorily with custom ASGI middleware, closing the PR for now