GetDiskInfo() uses timedValue to cache the disk info for one second.

timedValue behavior was recently changed to return an old cached value
when calculating a new value returns an error.

When a mount point is empty, GetDiskInfo() will return errUnformattedDisk,
timedValue will return cached disk info with unexpected IsRootDisk value,
e.g. false if the mount point belongs to a root disk. Therefore, the mount
point will be considered a valid disk and will be formatted as well.

This commit will also add more defensive code when marking root disks:
always mark a disk offline for any GetDiskInfo() error except
errUnformattedDisk. The server will try anyway to reconnect to those
disks every 10 seconds.