Ensure sparse files on windows during download

The file.Truncate call on windows will write the whole file
unless you set the sparse flag, leading to heavy I/O at the
beginning of download.  This should improve our
I/O behavior on windows and put less stress on the users disk.

server/sparse_windows.go

@@ -0,0 +1,16 @@
+package server
+
+import (
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+func setSparse(file *os.File) error {
+	return windows.DeviceIoControl(
+		windows.Handle(file.Fd()), windows.FSCTL_SET_SPARSE,
+		nil, 0,
+		nil, 0,
+		nil, nil,
+	)
+}