No listener.json for single-node mode (Fixes #3052) (#3108)

In FS or single-node XL mode, there is no need to save listener
configuration to persistent storage. As there is only one server, if it
is restarted, any connected listenBucketAPI clients were disconnected
and will have to reconnect - so there is nothing to actually store.

This incidentally solves #3052 by avoiding the problem.

cmd/s3-peer-client.go

@@ -32,6 +32,9 @@ type s3Peers struct {
 
 	mutex *sync.RWMutex
 
+	// Is single-node?
+	isDistXL bool
+
 	// Slice of all peer addresses (in `host:port` format).
 	peers []string
 }
@@ -53,6 +56,9 @@ func initGlobalS3Peers(eps []storageEndPoint) {
 
 	// Save new peers
 	globalS3Peers.peers = peers
+
+	// store if this is a distributed setup or not.
+	globalS3Peers.isDistXL = len(globalS3Peers.peers) > 1
 }
 
 func (s3p *s3Peers) GetPeers() []string {