datastore: use cached snapshot time string in path

When getting the `full_path` of a snapshot we did not use the cached
time string. By using it we avoid a call to the super-slow libc strftime.

This has some minor performance improvements of circa 7%. That is ~100ms
on my datastore with ~5000 snapshots.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>

pbs-datastore/src/backup_info.rs

@@ -415,7 +415,9 @@ impl BackupDir {
 
     /// Returns the absolute path for backup_dir, using the cached formatted time string.
     pub fn full_path(&self) -> PathBuf {
-        self.store.snapshot_path(&self.ns, &self.dir)
+        let mut path = self.store.base_path();
+        path.push(self.relative_path());
+        path
     }
 
     pub fn protected_file(&self) -> PathBuf {