Implement replace-disks for drbd8 devices

Implement replace-disks for drbd8 devices

This patch adds three modes of disk replacement for drbd8:
  - replace the disk on the primary node
  - replace the disk on the secondary node
  - replace the secondary node

It also adds some debugging code to backend.py and increments the
protocol version for the recent changes of the rpc layer.

Reviewed-by: imsnah