Disk quota computation redesign

[AndrewJGaut]

Currently, the disk quota for a user is updated using the update_disk_metadata function, which uses the update_user_disk_used function, which re-computes the user's disk quota usage from scratch by just summing the size of all bundles owned by the user.

That's slow. Instead, we'd like to just increment and decrement when necessary. [ADD IN BENCHMARKING STATS -- Some can be seen in the "Speed Up Rm Bundles" PR, since the speedup implemented was to replace update_disk_metadata with more efficinet code.)