Enable use of ionice for all handlers to alleviate IO load

On some workloads, a process creating backups should not become so
aggressive on resources that it prevents normal activity of a machine
from running. It is especially important when writing to disk.

One way to make backups have less impact on the main workload is to
force backups to a lower ionice level.

With the new ionicelevel configuration, it becomes possible for users to
activate the use of ionice and to define the level within the
best-effort class to better suit their needs.