Set thresholds for multipart uploading

[LeonidVas]

The default values of multipart_threshold and multipart_chunksize (8 MB) are currently used to upload packages.
Taking into account the typical file size of a package (not more than 10 MB) it looks like there is no point in batching the file into multiple files. I propose to set the threshold to 20 MB. I also propose to set such a threshold for the rws_backup.sh tool.

[Totktonada]

A suggestion from @Mons:

Set multipart_threshold to 500MB and multipart_chunksize to 100MB. The configuration is described here.

Don't forget to re-upload existing files.

[LeonidVas]

Don't forget to re-upload existing files.

Considering that only some older versions of tarantool take more than 8 MB, I don't think it makes sense.