Implement the multi_threading_mgr.h idea
See https://gitlab.isc.org/isc-projects/kea/wikis/designs/kea-multithreading-packet-processing
Note the design just gives a raw idea:
- a .cc file is required (no static initialization in a header)
- the constructor
- the class must be split into one returned by the instance static method which has only the get and a second which has both set and get but requires a dynamic cast to be used.
- of course a lot of comments