AthContainers: Allow customizing DV object deletion.

Add a mechanism to DataVector to customize how objects are deleted.
This is done by a polymorphic Deleter interface.

The goal here is to be able to use an alternate memory allocator
for the objects in a DV.  But for compatibility and other reasons,
we don't want to change the DV types themselves, which is why this
is done via a polymorphic interface.