feat(server): add h1 idle_timeout

[finnbear]

Motivation

Older browsers sometimes open 6 or more H1 connections and keep them alive and idle after resources are loaded, costing server resources and distracting from DoS attackers. In my specific case, DoS mitigation required limiting TCP connections per IP address, but limits had to be high to account for multiple legitimate H1 browsers on the same IP address.

Related: #1628
Related: #2355

Changes

• Added idle_timeout setting, optimized to avoid creating an additional timer per request
• Automatically send keep-alive: timeout=# header (deferred due to: Consider providing constants for keep-alive and proxy-connection header names. http#142)

Testing

• Fixed multiple pre-existing and broken, but relevant, tests
• Added a test with idle timeout
• Added a test without idle timeout (improves coverage)
• Tested in my app

# Cargo.toml

[patch.crates-io]
hyper = { git = "https://github.com/finnbear/hyper", branch = "idle_timeout" }
hyper-util = { git = "https://github.com/finnbear/hyper-util", branch = "idle_timeout" }

Future work

• Support in hyper-util (feat(server): add support for idle_timeout. hyper-util#146)
• Consider other timeouts (reading body, writing response)

[MarkusSintonen]

@sfackler / @seanmonstar is there a possibility on getting this PR reviewed? I am also interested in getting H1 idle timeouts available :)

[finnbear]

I am also interested in getting H1 idle timeouts available :)

If you want, you can use the TOML snippet in my PR description to avoid needing to wait for review :)