Bypass network in lock requests to local server (#4465)

This makes lock RPCs similar to other RPCs where requests to the local
server bypass the network. Requests to the local lock-subsystem may
bypass the network layer and directly access the locking
data-structures.

This incidentally fixes #4451.

cmd/server-mux_test.go

@@ -340,10 +340,11 @@ func TestServerListenAndServePlain(t *testing.T) {
 }
 
 func TestServerListenAndServeTLS(t *testing.T) {
-	_, err := newTestConfig(globalMinioDefaultRegion)
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
 	if err != nil {
 		t.Fatalf("Init Test config failed")
 	}
+	defer removeAll(rootPath)
 
 	wait := make(chan struct{})
 	addr := net.JoinHostPort("127.0.0.1", getFreePort())