Fix thread safety of TGenCollectionProxy's iterator creation.

The major change is that the utility routine returned by
TVirtualCollectionProxy::GetFunctionCreateIterators
now has the prototype:

typedef void (*CreateIterators_t)(void *collection, void **begin_arena, void **end_arena, TVirtualCollectionProxy *proxy);

i.e. adding a new pointer to the virtual collection proxy so
that in the case where the routine can not hard code all
the information (for example in the case of emulated collection
proxy) it can now get the information from a the CollectionProxy
without relying on the calls to PushProxy (which we are trying
to deprecate in general as it is somewhat thread adverse).