[#306] cmd/node: Switch health status on boot and shutdown

Implement HealthChecker on node app structure. Set health status to ONLINE
after node boot. Set health status to OFFLINE on shutdown.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/main.go

@@ -5,6 +5,7 @@ import (
 	"flag"
 	"log"
 
+	"github.com/nspcc-dev/neofs-node/pkg/services/private"
 	"github.com/nspcc-dev/neofs-node/pkg/util/grace"
 	"go.uber.org/zap"
 )
@@ -54,6 +55,8 @@ func bootUp(c *cfg) {
 	serveGRPC(c)
 	bootstrapNode(c)
 	startWorkers(c)
+
+	c.setHealthStatus(private.HealthStatus_ONLINE)
 }
 
 func wait(c *cfg) {