pgsql: tableam: Add table_get_latest_tid, to wrap heap_get_latest_tid.

tableam: Add table_get_latest_tid, to wrap heap_get_latest_tid.

This primarily is to allow WHERE CURRENT OF to continue to work as it
currently does. It's not clear to me that these semantics make sense
for every AM, but it works for the in-core heap, and the out of core
zheap. We can refine it further at a later point if necessary.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2e3da03e9ee4d6ee5cf0d1ffe0227fe6275397e1

Modified Files
--------------
src/backend/access/heap/heapam_handler.c | 1 +
src/backend/executor/nodeTidscan.c | 15 +++------------
src/backend/utils/adt/tid.c | 5 +++--
src/include/access/tableam.h | 18 ++++++++++++++++++
4 files changed, 25 insertions(+), 14 deletions(-)