Fix RefMutContainer drop bug

[jopemachine]

Existing RefMutContainer code also causes undefined behavior easily due to using unsafe code wrong.

For example,

>>> import rraft
>>> c = rraft.Config()
>>> r = c.make_ref()
>>> c = None
>>> r = None
(REPL spinning, not return)

After this patch,

>>> import rraft
>>> c = rraft.Config()
>>> r = c.make_ref()
>>> c = None
>>> r
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
rraft.DestroyedRefUsedError: Cannot use a destroyed object's reference!
>>> c = rraft.Config()
>>> r = c.make_ref()
>>> c = None
>>> r = None
>>> c
>>> r
>>> 

[jopemachine]

I'm not sure making shortcut type like below would be better here.

type RefTable<T> = HashMap<u64, Weak<Mutex<Option<NonNull<T>>>>>;

[jopemachine]

clippy said like below, so I decided to make an intermediate type like RefTable<T> mentioned before.

warning: very complex type used. Consider factoring parts into `type` definitions
  --> src/utils/reference.rs:16:11
   |
16 |     refs: Arc<Mutex<HashMap<u64, Weak<Mutex<Option<NonNull<T>>>>>>>,
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity
   = note: `#[warn(clippy::type_complexity)]` on by default