Support for TTreePerfStat and multiple TTree.

Currently TTreePerfStat can only monitor I/O operations on one TTree, this patch enables the use of multiple TTreePerfStats on different TTrees.

From Peter van Gemmerren and Roger Xiao.

Conflicts:
	core/base/inc/LinkDef3.h
	core/base/inc/RtypesCint.h
	core/base/inc/TThreadSlots.h