WIP: Backport fixes to build with LCG 95 + gcc8

For this, as discussed at the PAC. I merged the result of !800 (merged), then I cherry-picked !933 (merged) !795 (merged) !885 (merged) !1097 (merged) !1324 (merged) !811 (merged) !1276 (merged) !1124 (merged) !1295 (merged) !802 (merged) !818 (merged) !1144 (merged) !870 (merged) !805 (merged) !885 (merged) !803 (merged)

I also had to apply some minor fixed to the merge conflicts.

Important: I (temporarily) remove RichPhotonCounting (!1120 (merged)), because I do not know how to adapt it to !818 (merged).

Requires: Lbcom!347 (closed) LHCb!1792 (closed)