Following the implementation in TChain, I have succeded in enabling the TTree cache in Proof.
 However, for that I need a small addition in TTreeCache to be able to change also the 'fOwner'
 member, because on the workers we have to delete the previous tree, so the owner tree creating
 the cache is gone after the first file.
 In the attached patch I propose to add a second argument to TTreeCache::UpdateBranches, a boolean
 indicating whether to set also the ownership or not. Default is not, so we do not break  any existing
 usage of the call.

 With this, and other changes  in TEventIter that I am finalizing, I get for h1analysis with http files
 from my place and my single core machine

       TChain                   52s
       Proof 1 worker      57-62s

 i.e. a 10-20% overhead in Proof, which is what we expect without the local optimizations.


git-svn-id: http://root.cern.ch/svn/root/trunk@20794 27541ba8-7e3a-0410-8455-c3a389f83636