Correct type of waitMode variable in ExecInsertIndexTuples().

Correct type of waitMode variable in ExecInsertIndexTuples().

It was a bool, even though it should be CEOUC_WAIT_MODE. That's unlikely
to have a negative effect with the current definition of bool (char),
but it's definitely wrong.

Discussion: 20150812084351.GD8470@awork2.anarazel.de
Backpatch: 9.5, where ON CONFLICT was merged