From Jan-Fiete Grosse-Oetringhaus (ALICE) and me.
   Patch introducing the improved dataset manager developed by ALICE.
   The client side interface is basically unchanged, the biggest change is the renaming
   of TProof::CreateDataSet to TProof::RegisterDataSet.
   On the server side the relevant code has ben extracted from TProofServ and hidden
   behind an inteface class TProofDataSetManager describing the functionality needed by
   TProofServ, and an implementation class TProofDataSetManagerFile handling dataset
   meta data saved into ROOT files. TProofDataSetManagerFile is instantiated via the
   plugin manager and by default it handles the <sand_box>/datasets area.
   This separation is needed to allow loading implementation based on different backends;
   for example, ATLAS foresees to use a MySQL-based backend.
   A TWiki page with information and examples in on preparation.


git-svn-id: http://root.cern.ch/svn/root/trunk@23075 27541ba8-7e3a-0410-8455-c3a389f83636