First byte timeout

[DFINITYManu]

We have in hyper a HTTP1 header read timeout (which is strangely not available for HTTP?) and this works great... if the client sends at least one byte (or a newline?).

If the client never sends a single byte, and just holds the connection open, the timeout never triggers.

I suggest we either make the timeout trigger even when no bytes have been sent (make the timer start ticking the minute serve_connection() is called) or we introduce another field explicitly for first byte timeout.

This will harden hyper servers against clients who attempt to open and hog connections open without any intention of sending any data.

[seanmonstar]

Yes, several have indicated that a separate timeout makes sense: #3185 (comment)