rclone: avoid temp files

[Roman2K]

The rclone store is the only part we need temp files for. The rest works with stdin+stdout.

Waiting for rclone rcat:

• https://github.com/ncw/rclone/issues/230
• https://github.com/ncw/rclone/issues/1001

Then get rid of all temp files-related code (unless adding a pathcmd proc: command proc that takes path(s) as input).

[Blackskyliner]

rclone rcat is implemented as of rclone v1.38 tagged on 2017-09-30