libGeom: replace TThread::Lock() and Unlock() by their counterparts in std::mutex

Replace the TThread::Lock() and TThread::Unlock() invocations in libGeom by their counterparts in std::mutex.
A special case is TGeoManager, where other TThread methods like SelfId() are also used and depend on TThread::Lock()/Unlock() to be invoked. Thus, the replacement for all TThread methods used in TGeoManager should be done at the same time.