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/xl/xl.json

@@ -0,0 +1,57 @@
+{
+    "parts": [
+        {
+            "number": 1,
+            "size": 5242880,
+            "etag": "3565c6e741e69a007a5ac7db893a62b5",
+            "name": "object1"
+        },
+        {
+            "number": 2,
+            "size": 5242880,
+            "etag": "d416712335c280ab1e39498552937764",
+            "name": "object2"
+        },
+        {
+            "number": 3,
+            "size": 4338324,
+            "etag": "8a98c5c54d81c6c95ed9bdcaeb941aaf",
+            "name": "object3"
+        }
+    ],
+    "meta": {
+        "md5Sum": "97586a5290d4f5a41328062d6a7da593-3",
+        "content-type": "application\/octet-stream",
+        "content-encoding": ""
+    },
+    "minio": {
+        "release": "DEVELOPMENT.GOGET"
+    },
+    "erasure": {
+        "algorithm": "klauspost/reedsolomon/vandermonde",
+        "index": 2,
+        "distribution": [ 1, 3, 4, 2, 5, 8, 7, 6, 9 ],
+        "blockSize": 4194304,
+        "parity": 5,
+        "data": 5,
+        "checksum": [
+            {
+                "name": "object1",
+                "algorithm": "sha512",
+                "hash": "d9910e1492446389cfae6fe979db0245f96ca97ca2c7a25cab45805882004479320d866a47ea1f7be6a62625dd4de6caf7816009ef9d62779346d01a221b335c",
+            },
+            {
+                "name": "object2",
+                "algorithm": "sha512",
+                "hash": "d9910e1492446389cfae6fe979db0245f96ca97ca2c7a25cab45805882004479320d866a47ea1f7be6a62625dd4de6caf7816009ef9d62779346d01a221b335c",
+            },
+        ],
+    },
+    "stat": {
+        "version": 0,
+        "modTime": "2016-05-24T00:09:40.122390255Z",
+        "size": 14824084
+    },
+    "format": "xl",
+    "version": "1"
+}