Managed garbage collection

Determinate-nixd automatically schedules and manages garbage collection, based on a variety of criteria.

The general properties of garbage collection are:

• Retain at least 30GB of disk space free to allow for system updates
• Operate in a steady-state mode with between 5-20% disk space free

If the disk falls below 5% free, determinate-nixd enters an "urgent" garbage collection mode to immediately free disk space.