Amcheck verification of GiST and GIN

Hello world!

Few years ago we had a thread with $subj [0]. A year ago Heikki put a lot of effort in improving GIN checks [1] while
huntinga GIN bug. 
And in view of some releases with a recommendation to reindex anything that fails or lacks amcheck verification, I
decidedthat I want to review the thread. 

PFA $subj incorporating all Heikki's improvements and restored GiST checks. Also I've added heapallindexed verification
forGiST. I'm sure that we must add it for GIN too. Yet I do not know how to implement it. Maybe just check that every
entrygenerated from heap present in entry tree? Or that every tids is present in the index? 

GiST verification does parent check despite taking only AccessShareLock. It's possible because when the key discrepancy
isfound we acquire parent tuple with lock coupling. I'm sure that this is correct to check keys this way. And I'm
almostsure it will not deadlock, because split is doing the same locking. 

What do you think?

Best regards, Andrey Borodin.

[0] https://www.postgresql.org/message-id/flat/CAF3eApa07-BajjG8%2BRYx-Dr_cq28ZA0GsZmUQrGu5b2ayRhB5A%40mail.gmail.com
[1]
https://www.postgresql.org/message-id/flat/9fdbb584-1e10-6a55-ecc2-9ba8b5dca1cf%40iki.fi#fec2751faf1ca52495b0a61acc0f5532