This patch realizes the reorganizational changes outlined in ROOT-8513.
In details:
   - Move TExecutor into core/base
   - The multi-thread Tree processor TTreeProcessor is renamed TTreeProcessorMT
   - The TTree porcessing part of TProcessExecutor is moved to a new
     class TTreeProcessorMP situated under tree/treeplayer (the interface
     is under ROOT) as TTreeProcessorMT. The methods are renamed 'Process' instead
     of ProcTree for symmetry.
   - The internal classes depending on libTree and libRIO (TPoolPlayer, TPoolProcessor)
     are also moved to tree/treeplayer and reorganized in such a way to remove code
     duplication. Their names are also changed to reflect their task.
     The new code is the new files TMPWorkerTree{.h,.cxx}.
   - The internal clas TPoolWorker is renamed TMPWorkerExecutor to reflect its task.