Before this change, the Memory backend had the potential to deadlock under
certain conditions, if the ListR callback required locking the b.mu mutex. This
was the case with operations.Purge, because Memory has no Purge method, and the
fallback option does:
err = DeleteFiles(ctx, listToChan(ctx, f, dir))
which potentially starts removing objects before the listing has completed.
This change fixes the issue by batching all the entries before calling the
callback on them.
