pgsql: Add TupleTableSlotOps.is_current_xact_tuple() method

Add TupleTableSlotOps.is_current_xact_tuple() method

This allows us to abstract how/whether table AM uses transaction identifiers.
A custom table AM can use a custom slot, which may not store xmin directly,
but determine the tuple belonging to the current transaction in the other way.

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/0997e0af273d80add75bcf5616eee000d0a78397

Modified Files
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src/backend/executor/execTuples.c   | 79 +++++++++++++++++++++++++++++++++++++
src/backend/utils/adt/ri_triggers.c |  8 +---
src/include/executor/tuptable.h     | 21 ++++++++++
3 files changed, 101 insertions(+), 7 deletions(-)