Separate sum of key-value sizes for system keyspaces

[gm42]

The status output currently reports:

Data:
  Replication health     - Healthy
  Moving data            - 0.000 GB
  Sum of key-value sizes - 789.542 GB
  Disk space used        - 2.641 TB

However in some cases, system keyspaces can be cluttered with leftover blob granules (blog) from ongoing backups which did not succeed.

Having a separate entry for the sum of system key-value sizes would help diagnose such problems.