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/object_api_suite_test.go

@@ -715,6 +715,16 @@ func testNonExistantObjectInBucket(obj ObjectLayer, instanceType string, c TestE
 	}
 }
 
+// Check if error type is ObjectNameInvalid.
+func isErrObjectNameInvalid(err error) bool {
+	err = errorCause(err)
+	switch err.(type) {
+	case ObjectNameInvalid:
+		return true
+	}
+	return false
+}
+
 // Wrapper for calling testGetDirectoryReturnsObjectNotFound for both XL and FS.
 func (s *ObjectLayerAPISuite) TestGetDirectoryReturnsObjectNotFound(c *C) {
 	ExecObjectLayerTest(c, testGetDirectoryReturnsObjectNotFound)