Cache `Date` timestamps up to the minute

As pointed out by @Fishrock123 in https://github.com/http-rs/async-h1/pull/158/files#r530051517, Node.js caches timestamps for a minute, making it much easier to handle requests at a high volume. We should consider adopting this approach as well.

Start with the discussion on pull request 158 and trace where Date timestamps are generated in async-h1. The work is done when timestamp generation uses minute-level caching without changing the resulting HTTP date behavior; verify the existing relevant checks or benchmarks if the repository provides them.

Written by the indexing model from the issue text.