AESCCM race condition in callback mode

AESCCM has a race condition when executing any operation (bug located on decryption operations) in callback mode where the call to e.g. oneStepDecrypt can return MAC_INVALID if the callback fires before the function completes. This breaks the return semantics for the initial call in callback mode. We should consider buffering the return value in the function rather than storing it in the object where it can be overwritten.