Add new table_insert/update/delete() functions to stand in place of
heap_insert/update_delete(), and move the logic to dispatch the call to
AO, AOCO or external table function, depending on what kind of a table it
is. This moves the GPDB specific code out of the upstream functions,
making the diff vs upstream nicer to read.

PostgreSQL v12 introduced a new Table AM API. These new functions sit in
the same spot as the calls to the new API are made in v12. The signatures
don't quite match, but this is similar interface in spirit.
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>