Add sync mode for 'xl.json' (#6798)

xl.json is the source of truth for all erasure
coded objects, without which we won't be able to
read the objects properly. This PR enables sync
mode for writing `xl.json` such all writes go hit
the disk and are persistent under situations such
as abrupt power failures on servers running Minio.

cmd/storage-interface.go

@@ -47,6 +47,9 @@ type StorageAPI interface {
 	StatFile(volume string, path string) (file FileInfo, err error)
 	DeleteFile(volume string, path string) (err error)
 
+	// Write all data, syncs the data to disk.
+	WriteAll(volume string, path string, buf []byte) (err error)
+
 	// Read all.
 	ReadAll(volume string, path string) (buf []byte, err error)
 }