Teach amcheck to verify sibling links in all cases.

 Teach amcheck to verify sibling links in all cases.
 
 Teach contrib/amcheck's bt_index_check() function to check agreement
 between siblings links.  The left sibling's right link should point to a
 right sibling page whose left link points back to the same original left
 sibling.  This extends a check that bt_index_parent_check() always
 performed to bt_index_check().
 
 This is the first time amcheck has been taught to perform buffer lock
 coupling, which we have explicitly avoided up until now.  The sibling
 link check tends to catch a lot of real world index corruption with
 little overhead, so it seems worth accepting the complexity.  Note that
 the new lock coupling logic would not work correctly on replica servers
 without the changes made by commits 0a7d771f and 9a9db08a (there could
 be false positives without those changes).
 
 Author: Andrey Borodin, Peter Geoghegan
 Discussion: https://postgr.es/m/0EB0CFA8-CBD8-4296-8049-A2C0F28FAE8C@yandex-team.ru