AtomicBucket<T> unconditionally implements Send/Sync

In the affected versions of the crate, AtomicBucket<T> unconditionally implements Send/Sync traits. Therefore, users can create a data race to the inner
T: !Sync by using the AtomicBucket::data_with() API.
Such data races can potentially cause memory corruption or other undefined behavior.

The flaw was fixed in commit 8e6daab by adding appropriate Send/Sync bounds to the Send/Sync impl of struct Block<T> (which is a data type contained inside AtomicBucket<T>).