Improvments of COUNTD, COUNTD_MERGE and COUNTD_UNIQ_MERGE

[ne1r0n]

Thanks for adding the nice functions COUNTD_MERGE and COUNTD_UNIQ_MERGE, we found a great use for them. But now in order to add the -if modifier for these functions we have to fall back to using RAWSQLAGG_* functions. I see that this is already implemented for other aggregation functions, for example COUNT_IF or AVG_IF. Why not expand this list with COUNTD_MERGE_IF, COUNTD_UNIQ_MERGE_IF and COUNTD_IF functions?