Use Partition pointer adoption everywhere when adding items into a PartitionVector (#759726)

Replace all the current code which uses push_back() and insert() of a
local Partition object and gets it copy constructed into a
PartitionVector.  Instead allocate a Partition object on the heap and
adopt a pointer into the PartitionVector using push_back_adopt() and
insert_adopt().

Bug 759726 - Implement Partition object polymorphism