release unused objects from live object cache

* add a periodic sweep through the live object cache in the
  persistence module, releasing objects that have not been modified
  for a certain amount of time (except if there are active timers or
  intervals on the object)
* use 'ts' property of game objects to determine their "age", updated
  during construction and on write or del accesses on any (direct or
  indirect) property
* adjustments to existing tests (using the PBE mock) due to the
  different way the config is passed to the pers module