Allow table AM to store complex data structures in rd_amcache

Allow table AM to store complex data structures in rd_amcache

The new table AM method free_rd_amcache is responsible for freeing all the
memory related to rd_amcache and setting free_rd_amcache to NULL.  If the new
method is not specified, we still assume rd_amcache to be a single chunk of
memory, which could be just pfree'd.

Discussion: https://postgr.es/m/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com
Reviewed-by: Matthias van de Meent, Mark Dilger, Pavel Borisov
Reviewed-by: Nikita Malakhov, Japin Li