Fix memory leak of RefMutContainer when using infinite loop

[jopemachine]

I tested this patch on only just simple test like below example yet,

>>> import rraft
>>> c = rraft.Config()
>>> r1 = c.make_ref()
>>> r2 = c.make_ref()
>>> c = None
>>> r1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
rraft.DestroyedRefUsedError: Cannot use a destroyed object's reference!
>>> r2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
rraft.DestroyedRefUsedError: Cannot use a destroyed object's reference!
>>> c = rraft.Config()
>>> r1 = None
Object removed: '1'
>>> r2 = None
Object removed: '2'
>>> c
Config { id: 0, election_tick: 20, heartbeat_tick: 2, applied: 0, max_size_per_msg: 0, max_inflight_msgs: 256, check_quorum: false, pre_vote: false, min_election_tick: 0, max_election_tick: 0, read_only_option: Safe, skip_bcast_commit: false, batch_append: false, priority: 0, max_uncommitted_size: 18446744073709551615, max_committed_size_per_ready: 18446744073709551615 }
>>> r1
>>> r2

[jopemachine]

The Object removed: statement is for debugging in this PR.
FYI, It might be printed later than expected, for seeing the debugging statement in this case, we could use gc.collect() here.