Add log message about server releasing storage lock,

parallel with message about acquiring the lock.

src/ZEO/StorageServer.py

@@ -400,6 +400,7 @@ class ZEOStorage:
             self.locked = 0
             self.timeout.end(self)
             self.stats.lock_time = None
+            self.log("Transaction released storage lock")
         # _handle_waiting() can start another transaction (by
         # restarting a waiting one) so must be done last
         self._handle_waiting()