Simplify access to local node name (#11907)

The local node name is heavily used in tracing, create a new global 
variable to store it. Multiple goroutines can access it since it won't be
changed later.

cmd/globals.go

@@ -194,6 +194,9 @@ var (
 
 	globalEndpoints EndpointServerPools
 
+	// The name of this local node, fetched from arguments
+	globalLocalNodeName string
+
 	globalRemoteEndpoints map[string]Endpoint
 
 	// Global server's network statistics