Xl layer selfheal quorum2

* xl/selfheal: selfheal based on read quorum on GET

* xl: getReadableDisks() also returns whether self-heal is needed so that this info can be used by ReadFile/SelfHeal/StatFile.

* xl: trigger selfheal from StatFile.

xl-v1-utils.go

@@ -9,6 +9,7 @@ import (
 
 // Get parts.json metadata as a map slice.
 // Returns error slice indicating the failed metadata reads.
+// Read lockNS() should be done by caller.
 func (xl XL) getPartsMetadata(volume, path string) ([]fileMetadata, []error) {
 	errs := make([]error, len(xl.storageDisks))
 	metadataArray := make([]fileMetadata, len(xl.storageDisks))
@@ -38,6 +39,7 @@ func (xl XL) getPartsMetadata(volume, path string) ([]fileMetadata, []error) {
 //
 // Returns collection of errors, indexed in accordance with input
 // updateParts order.
+// Write lockNS() should be done by caller.
 func (xl XL) setPartsMetadata(volume, path string, metadata fileMetadata, updateParts []bool) []error {
 	metadataFilePath := filepath.Join(path, metadataFile)
 	errs := make([]error, len(xl.storageDisks))