Support DISTINCT and multiple expressions on aggregation functions

[erizocosmico]

According to MySQL documentation aggregation functions support DISTINCT clause and COUNT supports multiple expressions. We don't support neither.

I have been trying to implement the DISTINCT in a simple way by creating a DISTINCT aggregation node to pass to the aggregation functions instead but it's a bit complicated to do for aggregations this way so it ended up nowhere and this can't be done with query rewriting.

We should think more about how to implement this correctly.

Labeling as bug because right now it parses but produces wrong results.

[jlcanovas]

Just found this problem while using the tool and came to check if it was already reported. I see it was!

[erizocosmico]

This is already implemented and on master