restorer: pre-allocate files before loading chunks

internal/restorer/preallocate_other.go

@@ -0,0 +1,11 @@
+// +build !linux,!darwin
+
+package restorer
+
+import "os"
+
+func preallocateFile(wr *os.File, size int64) error {
+	// Maybe truncate can help?
+	// Windows: This calls SetEndOfFile which preallocates space on disk
+	return wr.Truncate(size)
+}