Re: Extended Statistics set/restore/clear functions.

Before I forget, one thing that I did find notable is that the mcv/expressions type importing code bypasses statatt_get_type() now, which means that there is only one caller for this function, and it could be moved back to being static.

Furthermore statatt_get_type() only exists because the two varietals of examine_attribute() are just slightly off what we need - the one in analyze.c will skip attributes with a target set to 0, and the one in extended_stats.c only handles expressions. So there might be some consolidation possible, but we'd have to change the code already being run for regular ANALYZE and up til now that's been something I haven't wanted to do.