get a recursive list of all file keys we do this so we can pick the correct page of results based on the limit and page query params for a recursive request.

GetKeysForDirLimit - recurse it creating s3 style keys for all the files in it (aka relative paths from that directory) send back a map of keys -> file sizes, and the whole directory's size.

NewFilesystemMachine - core functions used by files ending `state` which I couldn't think of a good place for.

FsConfig used to configure individual fsm.

a "filesystem machine" or "filesystem state machine".

state machinery.