Add UniqCount aggregation function

[cheb0]

Currently, we have Count function. It was the only function we had at the beginning in seq-db. Currently, it works as follows:
select count(foo) group by foo
which means we can only group and count over the same field.

Another aggregation function we have is Unique. It works like Count except it doesn't count unique values but only finds them.

New aggregation function UniqCount should allow to group by some field and aggregate over another field. It will work as follows:
select UniqCount(foo) group by bar