Updating key externals for CMSSW_7_4_X

- Move to new root (5.34.22)
- Move to new xrootd (4.0.4)
- Move to new boost (1.57.00)
- Clean a few headers.

This is a cleanup of https://github.com/cms-sw/cmsdist/compare/IB/CMSSW_7_3_X/stable...IB/CMSSW_7_3_X/next.