amcheck: Move common routines into a separate module

amcheck: Move common routines into a separate module

Before performing checks on an index, we need to take some safety
measures that apply to all index AMs. This includes:

* verifying that the index can be checked - Only selected AMs are
supported by amcheck (right now only B-Tree). The index has to be
valid and not a temporary index from another session.

* changing (and then restoring) user's security context

* obtaining proper locks on the index (and table, if needed)

* discarding GUC changes from the index functions

Until now this was implemented in the B-Tree amcheck module, but it's
something every AM will have to do. So relocate the code into a new
module verify_common for reuse.

The shared steps are implemented by amcheck_lock_relation_and_check(),
receiving the AM-specific verification as a callback. Custom parameters
may be supplied using a pointer.

Author: Andrey Borodin 
Reviewed-By: José Villanova 
Reviewed-By: Aleksander Alekseev 
Reviewed-By: Nikolay Samokhvalov 
Reviewed-By: Andres Freund 
Reviewed-By: Tomas Vondra 
Reviewed-By: Mark Dilger 
Reviewed-By: Peter Geoghegan 
Reviewed-By: Kirill Reshke 
Discussion: https://postgr.es/m/45AC9B0A-2B45-40EE-B08F-BDCF5739D1E1%40yandex-team.ru