Things to make different next time around (slog v3?)

• The whole defaulting to disable debug and trace in release build is probably a bad idea.
• The stuff that is behind flags now, should be standard.
• Drain::log to take &SendSyncRefUnwindSafeKV or something, instead of Owned.
• Make Logger a trait so people can extend it.
• https://github.com/slog-rs/slog/issues/169