pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/02eb07ea89d27f1d05a5055bf779042d2953b4e7

Modified Files
--------------
src/backend/access/heap/heapam_handler.c |  1 +
src/backend/utils/cache/relcache.c       | 11 +++--------
src/include/access/tableam.h             | 34 ++++++++++++++++++++++++++++++++
src/include/utils/rel.h                  | 10 ++++++----
4 files changed, 44 insertions(+), 12 deletions(-)