Temporary log levels

[RemiBardon]

Improving on #191, we could allow passing a parameter to the routes controlling log levels defining for how long a certain log level is enabled. This would allow features like a button "enable traces for 10 minutes" and would prevent people from forgetting they have traces enabled. This is what Cloudflare does with its "debugging" feature (I don't remember the exact name), where DNS caches are disabled for a fixed amount of time to help operators when configuring/debugging DNS.