[IMT] Let TTreeView support interleaved TBB task execution

When processing a TTree with TTreeProcessorMT, each thread creates
and uses its own TTreeView (by using a TThreadedObject<TTreeView>).

Until now, in the case of interleaved execution of tasks, the second
task could overwrite the first's TFile and TTree, causing crashes
when the first task resumed execution.

This commit modifies TTreeView so that it supports several tasks
opening and using different TFiles at the same time, solving the
issue described above.