default engine's file system client only lists _delta_log directory

[zachschuermann]

Do we want to assume the file system client only lists the log directory? Currently there's code which forces listing the table_root + _delta_log. We should either: (1) document this (say that this is a log-listing API), or (2) remove this assumption and let the filesystem client list any path in storage

[roeap]

This has been fixed in #699.