Add option to remove info objects for completed uploads

[Acconut]

The .info files/objects are needed by tusd for internal state keeping. Some users use them to load information when processing uploads. Other uses don't need them and they have to manually clean them up.

A feature would be handy to automatically remove the info object/file for completed uploads. This could be a setting and CLI flag that is passed to the data store, which then takes care of the removal in FinishUpload.