SPARK-2465. Use long as user / item ID for ALS

I'd like to float this for consideration: use longs instead of ints for user and product IDs in the ALS implementation.

The main reason for is that identifiers are not generally numeric at all, and will be hashed to an integer. (This is a separate issue.) Hashing to 32 bits means collisions are likely after hundreds of thousands of users and items, which is not unrealistic. Hashing to 64 bits pushes this back to billions.

It would also mean numeric IDs that happen to be larger than the largest int can be used directly as identifiers.

On the downside of course: 8 bytes instead of 4 bytes of memory used per Rating.

Thoughts?
I had understood all of this to be an `@Experimental` API, though it is not consistently marked. For example, `Rating` is experimental but its API is actually bound to this API here.