Set up IBs for CMSSW_7_4_THREADED_X

In cmssw:

I created branch CMSSW_7_4_THREADED_X from CMSSW_7_3_THREADED_X

https://github.com/cms-sw/cmssw/compare/CMSSW_7_4_THREADED_X...CMSSW_7_3_THREADED_X

then I did:
<pre>
git merge --no-ff -X ours origin/CMSSW_7_4_X
to make it up to date with CMSSW_7_4_X
</pre>

https://github.com/cms-sw/cmssw/compare/CMSSW_7_4_X...CMSSW_7_4_THREADED_X

in cmsdist:

CMSSW_7_4_THREADED_X uses branch IB/CMSSW_7_4_X/next