Cache/store sync status

[mazay]

Consider adding an optional cache for storing the sync status - list of files/s3 keys with checksums (maybe mtime) to speedup the initial sync which could take a lot of time depending on the data size. With cache restarts/reloads should be relatively fast.

Cache may also help with improving retries for failed files, e.g. save failed file in redid and retry in X s/m/h.