Remove deprecated pool_usage and pool_usage2 methods and add a new pool_usage
based on btrfs-debug-tree
Add pool usage model to the smart manager to keep track of pool usage over
time. Add south migration file for the change
Collect usage of all pools periodically as part of proc service. Note that atm,
the usage information is not coming from procfs but proc service is the most
logical place to add the collecting agent.
Add custom method to the Pool model to get current usage information. Update
the Pool info serializer with usage field. Clean up pool view to remove old
ways of getting pool usage to the new simpler way.
Bugfix: pool_usage is being called with old/wrong arguments in the view. Update
it to use the right method.
Use updated attributes from pool json to calculate usage

Use attributes total and used from pool json to calculate usage
Remove commented code

Remove commented code