fs: Re-implement object layer to remember the fd (#3509)

This patch re-writes FS backend to support shared backend sharing locks for safe concurrent access across multiple servers.

cmd/server-mux_test.go

@@ -109,18 +109,20 @@ func dial(addr string) error {
 
 // Tests initializing listeners.
 func TestInitListeners(t *testing.T) {
+	portTest1 := getFreePort()
+	portTest2 := getFreePort()
 	testCases := []struct {
 		serverAddr string
 		shouldPass bool
 	}{
 		// Test 1 with ip and port.
 		{
-			serverAddr: "127.0.0.1:" + getFreePort(),
+			serverAddr: "127.0.0.1:" + portTest1,
 			shouldPass: true,
 		},
 		// Test 2 only port.
 		{
-			serverAddr: ":" + getFreePort(),
+			serverAddr: ":" + portTest2,
 			shouldPass: true,
 		},
 		// Test 3 with no port error.