Re: Extended Statistics set/restore/clear functions.

Paquier's response got sidetracked because of an errant subject line change, so I will try to recap:

* pg_dump code changes no longer seem as bad on second look

* proceed with breakout per off-list discussion

in that off-list discussion I proposed (though I was mostly echoing what I thought Paquier wanted):

1. pg_ndistinct output function change.
2. pg_ndistinct input function addition.
3. pg_dependencies output function change
4. pg_dependencies input function
5. Expose attribute statistics function and rename them attstat_* or statatt_*   (edit: and fix lack of comments on the enums and arrays)
6. pg_restore_extended_stats
7. pg_dump with no ability to fetch old-format pg_ndistinct/pg_dependences. (edit: and fix inherited bug)
8. pg_dump working back as far as possible

Given that the pg_dump code no longer seems as bad, and Tomas is very much in support of it, I've opted not to split out steps 7/8.