need $addToSetIfNotLast operator

This would allow for unique if not last entry into arrays.

Preventing:

```

['one', 'one', 'two', 'three']

```

But allowing:

```

['one', 'two', 'three', 'one']

```

$addToSet was created because you cannot be expected to use locks at the database level when there is request logic on the server that depends on TCP because a the TCP timeout being able to block everything else.

It's reasonable to expect that you would want unique if not last entries into an array.