storagedriver/s3: Optimized Walk implementation + bugfix

Optimized S3 Walk impl by no longer listing files recursively. Overall gives a huge performance increase both in terms of runtime and S3 calls (up to ~500x).

Fixed a bug in WalkFallback where ErrSkipDir for was not handled as documented for non-directory.

Signed-off-by: Collin Shoop <cshoop@digitalocean.com>

registry/storage/driver/inmemory/driver.go

@@ -245,7 +245,7 @@ func (d *driver) URLFor(ctx context.Context, path string, options map[string]int
 }
 
 // Walk traverses a filesystem defined within driver, starting
-// from the given path, calling f on each file
+// from the given path, calling f on each file and directory
 func (d *driver) Walk(ctx context.Context, path string, f storagedriver.WalkFn) error {
 	return storagedriver.WalkFallback(ctx, d, path, f)
 }