[Performance Idea] Cache file mtimes for mapping step

[CodyCBakerPhD]

During the mapping step; if the mapped dandiset/dandifile exists and the mtime of the corresponding binned file has not changed (binning has not been rerun) since last mapping, then we can skip the mapping step for that asset since it would not produce a different result

All we need to do is cache a mapping of binned file paths to mtimes of previous run