FileWalker implements methods to walk over pieces in
in a storage directory.
This is just a refactor to separate filewalker functions
from pieces.Store. This is needed to simplify the work
to create a separate filewalker subprocess and reduce the
number of config flags passed to the subprocess.
You might want to check https://review.dev.storj.io/c/storj/storj/+/9773
Change-Id: I4e9567024e54fc7c0bb21a7c27182ef745839fff
