Halt cluster in case of simultaneus device loss

[sirio81]

When a device get's disconnected, the copy of its objects may be on any other node device.
If more devices fails in the same period of time (during recovery), and the number of the lost devices is equal the redundancy limit, sheepdog should halt the cluster waiting for the administrator intervention.

E.g.
-c 2 cluster halt if 2 devices die at the same time
-c 3 cluster halt if 3 devices die at the same time
-c 4:2 cluster halt if 3 devices die at the same time

This way it may be possible to restore the missing objects keeping intact the vdi content (filesystem consistency).