Replace `ThreadPoolSvc::m_tbbgc` raw pointer with `unique_ptr`

The following discussion from !887 (merged) should be addressed:

• @graven started a discussion: (+2 comments)

  why not make this std::unique_ptr<tbb::global_control>? In that case, not only is not needed to explicitly initialize it with nullptr, but also there is no need for an explicit destructor...