Don't output whole timers map; hash headers in rust

[vlad-ivanov-name]

• The auth timers map can become quite big due to reasons described in Limit size of auth cache keyed by HTTP password #1412; don't output the whole map to tracing, just its size
• Hash headers in rust instead of calling C lib. While initially I tried this as a fix for a different problem that didn't work, I decided to keep it, just because sha256 is a bit more up-to-date on security: since we hash without any extra salt and the format of source string is known, hypothetically rainbow tables could be used, etc. Using sha256 will make it more difficult.