implement out file storage

[felixge]

We need a way to locally store files, which may be coming in as partial chunks. These chunks may overlap, which must cause no problems if the overlapping parts hold the same data.

My idea: For the first request create a file id, then create tus_data/<fileId> and tus_data/<fileId>.log. The first file has the full size of the expected is empty, and gets filled as data comes in. The second file is an append-only log that lists all chunks of the file that have been received. The format for this could be new-line separated JSON.

[felixge]

Done.