Refine debug logging

We used to rely on cached_xlog_location and replaced_xlog_location when showing a debug message about pod update prevention, but that produced false positives, i.e.
```
2024-06-06 11:34:36,308 DEBUG: prevented pod update, keeping cached xlog value for up to -1 seconds
```

Make sure we only show this message when we actually used cached xlog, no updates happen, but they should have happened,
since cached value and received member value do not match.

patroni/dcs/kubernetes.py

@@ -1344,21 +1344,22 @@ def touch_member(self, data: Dict[str, Any]) -> bool:
         replaced_xlog_location: Optional[str] = data.get('xlog_location', None)
         cached_xlog_location, last_updated = self._get_cached_xlog_location()
         now = time.time()
+        use_cached_xlog = False
         if last_updated is not None and last_updated + self._xlog_cache_ttl > now:
             if cached_xlog_location is not None and replaced_xlog_location is not None:
                 data['xlog_location'] = cached_xlog_location
+                use_cached_xlog = True
         elif replaced_xlog_location is not None:
             # location cache expired
             self._set_cached_xlog_location(replaced_xlog_location)
-            replaced_xlog_location = None
         ret = member and pod_labels is not None\
             and all(pod_labels.get(k) == v for k, v in role_labels.items())\
             and deep_compare(data, member.data)
 
         if not ret:
             # if we move forward with an update anyway, make sure to write the actual
             # value for the xlog, and not the stale cached value.
-            if replaced_xlog_location is not None:
+            if use_cached_xlog:
                 self._set_cached_xlog_location(replaced_xlog_location)
                 data['xlog_location'] = replaced_xlog_location
             metadata = {'namespace': self._namespace, 'name': self._name, 'labels': role_labels,
@@ -1367,7 +1368,7 @@ def touch_member(self, data: Dict[str, Any]) -> bool:
             ret = self._api.patch_namespaced_pod(self._name, self._namespace, body)
             if ret:
                 self._pods.set(self._name, ret)
-        elif cached_xlog_location != replaced_xlog_location and last_updated is not None:
+        elif use_cached_xlog and cached_xlog_location != replaced_xlog_location:
             logger.debug("prevented pod update, keeping cached xlog value for up to %d seconds",
                          (last_updated + self._xlog_cache_ttl - now))