Add RAND_INTEGER function, which returns a random integer modulo N

Add a RANDOM function.
It is not standard SQL, but most DBMSs have one, and it be useful.

It will also allow us to better test non-deterministic expressions.

I propose that:

• RANDOM(n) returns an integer between 0 and n - 1 inclusive.
• There is no facility to specify a seed at this time.
• The random number generator is randomly seeded at query start time based on (say) the method used by Java's new Random(); if you run the query twice, you will (probably) not get the same results.
• If you call the function twice in the same row, it will (probably) return different results.