XL: Rename, cleanup and add more comments. (#1769)

- xl-v1-bucket.go - removes a whole bunch of code.
- {xl-v1,fs-v1}-metadata.go - add a lot of comments and rename functions
   appropriately.

docs/backend/fs/format.json

@@ -0,0 +1,4 @@
+{
+  "format": "fs",
+  "version": "1"
+}