Let table AM insertion methods control index insertion

Let table AM insertion methods control index insertion

Previously, the executor did index insert unconditionally after calling
table AM interface methods tuple_insert() and multi_insert().  This commit
introduces the new parameter insert_indexes for these two methods.  Setting
'*insert_indexes' to true saves the current logic.  Setting it to false
indicates that table AM cares about index inserts itself and doesn't want the
caller to do that.

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