Support COUNT(*) for Aggregate COUNT function

[bobbai00]

Currently for aggregate operator, when selecting COUNT as the aggregation function, an attribute must be given. But in some use cases, there are no suitable attribute to select (e.g. there is no "primary key alike" attribute but users may want to count the occurance).

So a * attribute to support COUNT(*) should be useful.

[yunyad]

We decided not to proceed with it for now for several reasons:

1. The aggregate functionality includes many other options besides count, so it seems unreasonable to add it there.
2. Since this is not urgent, we can find a better position for this functionality.