Have a separate routing bit writer per stream

In the online world, all raw events go into one "pipe". They are later split into streams according to the routing bits. An event that is selected in N (>1) streams will thus get N raw events with slightly different content (e.g. different raw banks). A separate routing bit writer for each stream is thus needed in order to prevent having all of the N raw events go into each of the N streams.

• Add/modify a test so that we have some coverage for this

This is a follow-up from the February FEST

Goes with MooreAnalysis!73 (merged)

/cc @jonrob @nskidmor