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/handler-utils.go

@@ -541,7 +541,7 @@ func errorResponseHandler(w http.ResponseWriter, r *http.Request) {
 // gets host name for current node
 func getHostName(r *http.Request) (hostName string) {
 	if globalIsDistErasure {
-		hostName = GetLocalPeer(globalEndpoints)
+		hostName = globalLocalNodeName
 	} else {
 		hostName = r.Host
 	}