simplify kvdb-rocksdb restore function

[debris]

fs-swap crate: https://github.com/debris/fs-swap

[tomusdrw]

fs-swap doesn't look atomic. Maybe better randomize the filename witch each run a bit?

[debris]

pr is ready for another review. fs-swap is now atomic, but it may fail if:

a) osx is not using APFS (which is the default)
b) windows directories or on different volumes

in these cases we're trying to do nonatomic swap

[andresilva]

The changes in parity look good to me. Regarding fs-swap:

• Linux: you pass libc::AT_FDCWD to renameat2 shouldn't these be file descriptors to the paths we want to swap?

[debris]

from http://man7.org/linux/man-pages/man2/rename.2.html

If oldpath is relative and olddirfd is the special value AT_FDCWD,
then oldpath is interpreted relative to the current working directory
of the calling process (like rename()).

If I understand this correctly, by passing AT_FDCWD, the paths are being resolved as in rename(), so exactly as we want

[andresilva]

Makes sense, we want to resolve relative paths using the cwd of the process.

[debris]

already merged as a part of #8712