Increasing storage pool width

The width of all vaults on this storage pool increase. The rebuilder populates the newly added devices; the threshold is not changed. If a storage pool doesn't have any vaults, delete the pool and create a new pool with the wanted width.

After expansion, all the Slicestor devices that are associated with this Storage Pool, including the Accesser devices must be restarted. The Slicestor devices MUST be restarted before the Accesser devices. Vault size calculations might fluctuate after expansion due to averaging and rebuilding of the new slices, but will return to the original value.