Maintenance loops

Nothing in MooseFS acts on a policy at the moment the policy is set. Background passes apply the policy later. The period of these passes is the delay between setting something and seeing its effect.

The chunk loop and the file loop covers the two scans that the Master Server runs over all files and all chunks: what each scan does in a pass, and how long a pass takes. Chunkserver housekeeping loops covers the work that a Chunkserver starts by itself on its own disks, and which part of that work is visible to the Master Server. Client-side background work covers the threads inside a mount that keep the session alive, push buffered writes out, and read ahead. The page covers these threads because they decide when written data really leaves the machine.