Use a dedicated temporary folder for downloads

Currently, we are downloading files into a temporary hidden file.
It is causing a lot of false FS events, generating significative noise, thus complexifying the synchronization.

We could use a dedicated temporary folder that will be outside of any sync roots.

To generate less noise while downloading a file, we will introduce a new integer option: tmp_file_limit (defaults to 10, file size in MiB). Files lighter than this limit will be written at once to the file rather than chunk by chunk.

Benefits:

• no more false FS events;
• no more temporary .nxpart files;
• files smaller than tmp_file_limit will be downloaded in one go in-memory, reducing disk I/O.