As part of fixing the IO priority of filewalker related
processes such as the garbage collection and used-space
calculation, this change allows the initial used-space
calculation to run as a separate subprocess with lower
IO priority.
This can be enabled with the `--storage2.enable-lazy-filewalker`
config item. It falls back to the old behaviour when the
subprocess fails.
Updates https://github.com/storj/storj/issues/5349
Change-Id: Ia6ee98ce912de3e89fc5ca670cf4a30be73b36a6
