Migration of HLT to use TrackListMerger (from SimpleTrackListMerger)

Migration of HLT to use TrackListMerger (from SimpleTrackListMerger).
No physics changes!

This PR gets rid of annoying messages such as:
```
%MSG-w SimpleTrackListMerger:  SimpleTrackListMerger:hltL3TkTracksFromL2@ctor  \
24-Nov-2014 07:02:07 CET pre-events
this class is obsolete, please use TrackListMerger
%MSG
```

This is a forward port of #6568 into CMSSW_7_4_X.