feat: atomic writing

[Blu-J]

About

Renames are safer and are more atomic than just writing to a file.

Why not just using a tmp file:

• What if the service is writing it's own .tmp files and the file names conflict? we could clobber them and cause weird bugs in the service
• What if the service has behaviors that depend on stat-ing a directory? The new file could cause it to try to do something invalid

Proof it works

Nothing needs to change on the api side. Thus the old tests still working is proof that it works