XL: Implement strided erasure distribution. (#1772)

Strided erasure distribution uses a new randomized
block distribution for each Put operation. This
information is captured inside `xl.json` for subsequent
Get operations.

erasure.go

@@ -24,10 +24,11 @@ type erasure struct {
 	DataBlocks   int
 	ParityBlocks int
 	storageDisks []StorageAPI
+	distribution []int
 }
 
 // newErasure instantiate a new erasure.
-func newErasure(disks []StorageAPI) *erasure {
+func newErasure(disks []StorageAPI, distribution []int) *erasure {
 	// Initialize E.
 	e := &erasure{}
 
@@ -46,6 +47,9 @@ func newErasure(disks []StorageAPI) *erasure {
 	// Save all the initialized storage disks.
 	e.storageDisks = disks
 
+	// Save the distribution.
+	e.distribution = distribution
+
 	// Return successfully initialized.
 	return e
 }