Several changes in the handling of TRef to support the following use case:

An array of objects is written to a file but the TRefs
referencing these objects are not written to the file.
In a next session, these objects are read in memory
and new TRefs point to these objects and are written
to a separate file. In a third session both files
are connected.
To get this scenario working, one must use a dummy TRef
set to all objects that can be referenced in the next session.


git-svn-id: http://root.cern.ch/svn/root/trunk@4282 27541ba8-7e3a-0410-8455-c3a389f83636