Implement and use virtual Partition copy constructor clone() (#759726)

Final step for full polymorphic handling of Partition objects is to
implement a virtual copy constructor.  C++ doesn't directly support
virtual copy constructors, so instead use the virtual copy constructor
idiom [1].  (Just a virtual method called clone() which is implemented
in every polymorphic class and creates a clone of the current object and
returns a pointer to it).

Then replace all calls to the (monomorphic) Partition object copy
constructor throughout the code, except in the clone() implementation
itself, with calls to the new virtual clone() method "virtual copy
constructor".

Also have to make the Partition destructor virtual too [2][3] so that
the derived class destructor is called when deleting using a base class
pointer.  C++ supports this directly.

[1] Wikibooks: More C++ Idioms / Virtual Constructor
    https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Virtual_Constructor

[2] When to use virtual destructors?
    http://stackoverflow.com/questions/461203/when-to-use-virtual-destructors

[3] Virtuality
    Guideline #4: A base class destructor should be either public and
    virtual, or protected and nonvirtual
    http://www.gotw.ca/publications/mill18.htm

Bug 759726 - Implement Partition object polymorphism

SQUASH: When first using pointers to Partition and calling delete