GitLab

os.Rename documentation says: "OS-specific restrictions may apply when
oldpath and newpath are in different directories." On Unix, this means
we can't rename across devices. On Plan 9 however, the functionality is
even more limited: cross-directory renames are not allowed at all.

Add a wrapper around os.Rename for Plan 9, which will copy the file if
we're renaming across directory. All tests seems to pass.

(Aside: I also had to write this wrapper to get go-git working on Plan 9:
https://github.com/go-git/go-billy/blob/v5.0.0/osfs/os_plan9.go#L27
but I notice few issues with that one.)

Fixes #86

(and make Close threadsafe)

This now means that the datastore needs to be properly closed, to clean up
the background thread.

Since Go 1.5, os.Rename is consistent across platforms

This also means that it is no longer possible to report the estimate time
left.