ddfs master: Introduce the KB macro.

This is used for converting the sizes from bytes to kilobytes and is 1024.
ddfs gc: Use weights for choosing the nodes to host the replicas.
ddfs master: Use a better utilization for abosolute space.

We are now using -1 * FreeSpace instead of 1 / FreeSpace.  The benefit is that
it is a meaningful value now and lets us set the threshold in absolute terms
too.
ddfs gc: Cleanup and organize some comments.
ddfs master: Move some of the rebalance functions to the ddfs_rebalance module.
ddfs rebalance: Introduce is_balanced and less functions.

These functions are used to abstract the utility type from the gc module.