amcheck verification for GiST

Hi, hackers!

Here's the patch with amcheck functionality for GiST.

It basically checks two invariants:
1. Every internal tuple need no adjustment by tuples of referenced page
2. Internal page reference or only leaf pages or only internal pages

We actually cannot check all balanced tree invariants due to concurrency reasons some concurrent splits will be visible
astemporary balance violations. 

Are there any other invariants that we can check?

I'd be happy to hear any thought about this.

Best regards, Andrey Borodin.