Counterblock doesn't log shutdown signal

[unsystemizer]

Unlike counterparty-server, counterblock doesn't log received shutdown signal (e.g. what you'd expect to see in its log after sudo sv stop counterblock-testnet.)
It's a minor thing, but tailing counterblock server log doesn't show when it was shutdown - it just ends.
That makes it a bit harder to figure out what it does during troubleshooting.

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