ENH: Uploads are no longer concatenated

The uploads handler appends every chunk of transmitted data into an
"in-construction" file. This avoids the overhead caused by first writing all
the chunks and then iterating over every chunk to construct the final file.

An important thing to note with this patch is that uploads will not be 
resumable as chunks are appended into the final file as they come and thus
there's no way to figure out which chunks were successfully transmitted and
which weren't.

Fixes #599