Re: brin autosummarization -- autovacuum "work items"

Here's a version of this patch which I consider final.

Thanks for your tips on using DSA.  No crashes now.

I am confused about not needing dsa_attach the second time around.  If I
do that, the dsa handle has been 0x7f'd, which I don't understand since
it is supposed to be allocated in TopMemoryContext.  I didn't dig too
deep to try and find what is causing that behavior.  Once we do, it's
easy to remove the dsa_detach/dsa_attach calls.

I added a new SQL-callable function to invoke summarization of an
individual page range.  That is what I wanted to do in vacuum (rather
than a scan of the complete index), and it seems independently useful.

I also removed the behavior that on index creation the final partial
block range is always summarized.  It's pointless.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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