xl: Introduce new blocking writer to make CreateFile atomic. (#1362)

Creates a new write closer that must be released
by the read consumer. This is necessary so that
while commiting the underlying writers in erasure
coding we need to make sure we reply success only if
we have committed to disk.

This in turn also fixes plethora of bugs related to
subsequent PutObject() races with namespace locking.

This patch also enables most of the tests, other than
ListObjects paging which has some issues still.

Fixes #1358, #1360

object-api.go

@@ -287,6 +287,8 @@ func (o objectAPI) ListObjects(bucket, prefix, marker, delimiter string, maxKeys
 			})
 		}
 	}
+
+	// Default is recursive, if delimiter is set then list non recursive.
 	recursive := true
 	if delimiter == slashSeparator {
 		recursive = false
@@ -298,6 +300,7 @@ func (o objectAPI) ListObjects(bucket, prefix, marker, delimiter string, maxKeys
 	if maxKeys == 0 {
 		return ListObjectsInfo{}, nil
 	}
+
 	result := ListObjectsInfo{IsTruncated: !eof}
 	for _, fileInfo := range fileInfos {
 		// With delimiter set we fill in NextMarker and Prefixes.