Calling stat() (really, lstat()) on every file during a directory walk
is the step that takes up the most time. Furthermore, not all directory
walk uses _need_ to have a stat done on every file. Therefore, in this
commit we avoid doing the stat at the lowest level of
walkNamespaceInPath. The stat will still be done when it is requested,
with the Stat() method on the blobInfo object.
The major upside of this is that we can avoid the stat call on most
files during a Retain operation. This should speed up garbage collection
considerably.
The major downside is that walkNamespaceInPath will no longer
automatically skip over directories that are named like blob files, or
blob files which are deleted between readdir() and stat(). Callers to
walkNamespaceInPath and its variants (WalkNamespace,
WalkSatellitePieces, etc) are now expected to handle these cases
individually.
Thanks to forum member Toyoo for the insight that this would speed up
garbage collection.
Refs: https://github.com/storj/storj/issues/5454
Change-Id: I72930573d58928fa25057ed89cd4ec474b884199
ed7c82439d