Slog runtime switchable drain

AtomicSwitch allows swapping drain that it wraps atomically, race-free, in runtime. This can be useful eg. for turning on debug logging in production.

See signal.rs example.

slog-atomic - Atomic run-time controllable drain for slog-rs

Using slog-atomic you can create a slog::Drain that can change behavior in a thread-safe way, in runtime. This is useful eg. for triggering different logging levels from a signal handler.