change lineitem.quantity from integer to double

[ilfrin]

This change is required to conform to the TPC-H spec. The 2.17.1 version
states that the lineitem.l_quantity field should be of decimal type (page
17), as opposed to the currently implemented integer.

I came across this when testing Presto with decimal on TPC-H queries and checking the results.

Not sure if I changed all the necessary bits.

cc @dain

[dain]

IIRC, quantity is always a whole number so BIGINT is more appropriate until DECIMAL lands in Presto.

[ilfrin]

We can look at this as you say. Then again my original intent was for the schema to conform to the spec. I understand that the generator produces only integers, but quantity in theory may be a fraction as well, which I believe was the intent of the spec. I don't feel strongly either way, so we can land this after decimal lands in Presto, of you prefer that.

Then again, this being a separate airlift project was about abstracting from Presto, so why should we connect that this way :)

This isn't a show stopper, so we can wait.