Use thread memory pools in TrackEventFitter

The following discussion from !2350 (merged) should be addressed:

• @graven started a discussion: (+9 comments)

  could also add EventContext const& evtCtx to the list of transformer arguments, and then do:

     auto memResource = LHCb::getMemResource( evtCtx );   
  
     std::vector<std::reference_wrapper<LHCb::Track>, LHCb::Allocators::EventLocal< std::reference_wrapper<LHCb::Track> >> fittingTracks{ memResource };
     fittingTracks.reserve( tracksCont.size() );
  
     std::vector<double, LHCb::Allocators::EventLocal<double>> qopBeforeVector{memResource}; 
     qopBeforeVector.reserve( tracksCont.size() );