Make argument to CleanCacheUnlocked mandatory

Not passing the argument means it has the value None. Iterating None
doesn't work:
  >>> "123" in None
  Traceback (most recent call last):
    File "<stdin>", line 1, in ?
  TypeError: iterable argument required

Hence I rename it to "exclude" instead of "exceptions", which may be
confusing, and make it mandatory. If one wants to clean all cache
entries, an empty list can be passed.

Reviewed-by: iustinp