Change Database::compare_and_swap to atomically replace the target

- Currently, the file system code writes the data to a temp file,
    removes the target file, and then renames the temp file to the
    target file.

  - This has two problems.  1.) If the rename fails, then the old
    version is also gone.  2.) A reader (e.g., NGINX) will return a
    404 after the old file has been removed, but before the file has
    been renamed.

  - Since tmp.persist() atomically replaces the target file, there is
    no need to remove the file.

src/database/fs.rs

@@ -171,15 +171,6 @@ impl Database for Filesystem {
                     .tempfile_in(dir)?;
                 tmp.write_all(new)?;
 
-                if target.is_file() {
-                    if old.is_some() {
-                        remove_file(target.clone())?;
-                    } else {
-                        return Err(
-                            format!("stray file {}", target.display()).into()
-                        );
-                    }
-                }
                 let _ = tmp.persist(ensure_parent(&target)?)?;
 
                 // fix permissions to 640