import/export daemon: Record amount of data transferred

This reports the amount of data transferred and the throughput (averaged
over 60 seconds) to the master daemon. While not yet fully implemented,
once the export scripts report the expected data size, we can even provide
an ETA and percentage.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>