with-temporary-store-file suffers from race conditions - see attached
patch commit message for details.

This addresses a problem that it's very likely nobody has run into yet,
due to the sheer size of the tempfile namespace, but that could
potentially cause serious issues, including store corruption.

The new procedure used to resolve this (restore-to-temp-store-file) will
be of use in the guile-daemon anyway.

- reepca