Fix unpack cache reclaimer (#3605)

PBENCH-1318

The reclaimer defaulted to 20%, which is inappropriate for an unpack reclaim
where we want to free just enough for the unpacked dataset size.

Also, to help diagnose, add the last referenced cache date to the reclaim log
message.

lib/pbench/server/cache_manager.py

@@ -1,5 +1,6 @@
 from collections import deque
 from dataclasses import dataclass
+from datetime import datetime
 from enum import auto, Enum
 import errno
 import fcntl
@@ -1524,7 +1525,7 @@ def delete(self, dataset_id: str):
 
         self._clean_empties(tarball.controller_name)
 
-    def reclaim_cache(self, goal_pct: float = 20.0, goal_bytes: int = 0) -> bool:
+    def reclaim_cache(self, goal_pct: float = 0.0, goal_bytes: int = 0) -> bool:
         """Reclaim unused caches to free disk space.
 
         The cache tree need not be fully discovered at this point; we can
@@ -1620,7 +1621,10 @@ def reached_goal() -> GoalCheck:
                 target = None
             error = None
             try:
-                self.logger.info("RECLAIM: removing cache for {}", name)
+                ts = datetime.fromtimestamp(candidate.last_ref)
+                self.logger.info(
+                    "RECLAIM: removing cache for {} (referenced {})", name, ts
+                )
                 with LockManager(cache_d / "lock", exclusive=True, wait=False):
                     try:
                         if target: