HLT1 data-oriented event model - general discussion

cc @gligorov @graven @sstahl @rmatev @sponce @jonrob

General discussion on the HLT1 data-oriented event model. Ideas, plans, big picture things should be discussed here, and keep the MRs only for the code.

Please refer the following merge requests: LHCb!1717 (closed) + Lbcom!315 (closed) + !1391 (closed) + Brunel!626 (closed)