feat(debug): Add SIGUSR1 handler to dump state info

This produces output like this in the log:

Mar 06 00:57:18 localhost fleet[10709]: I0306 00:57:18.044789 10709 fleet.go:90] Dumping server state
Mar 06 00:57:18 localhost fleet[10709]: {"Agent":{"State":{"Offers":{},"Conflicts":{"hello.service":[],"ping.service":[],"pong.service":[]},"Bids":[],"Peers":{"ping.service":["pong.service"]}}}}
Mar 06 00:57:18 localhost fleet[10709]: I0306 00:57:18.045274 10709 fleet.go:105] Finished dumping server state