This PR is inspired by a recent code review comment on a PR that
introduced new functions to the net_processing module. The point of the
discussion was basically that whenever we pass something not by value
(in the concrete example it was about CNode* and CConnman*) we should
either use

    a pointer (CType*) with null pointer check or
    a reference (CType&)

To keep things simple, this PR for a first approach

    only tackles CNode* pointers
    only within the net_processing module, i.e. no changes that would
need adaption in other modules
    keeps the names of the variables as they are

I'm aware that PRs like this are kind of a PITA to review, but I think
the code quality would increase if we get rid of pointers without
nullptr check -- bloating up the code by adding all the missing checks
would be the worse alternative, in my opinion.

Possible follow-up PRs, in case this is received well:

    replace CNode pointers by references for net module
    replace CConnman pointers by references for net_processing module
    ...

ninja all check-all