static deltas: Process each part as soon as its done

Directly when we allocate a new part we finish the old one,
writing the compressed data to a temporary file and generating
the delta header for it.

When all these are done we loop over them and collect the headers,
sizes and either copy the tempfile data into the inlined superblock
or link the tempfiles to disk with the proper names.

Closes: #1309
Approved by: cgwalters