This is a follow-up to the recently merged PR #19053, replacing two more
types of one more type of pointer (CConnman) by references to increase
the code quality -- pointers should either check for nullptr or be
replaced by references, and the latter strategy seems to be more
reasonable.

Again, to keep the review burden managable, the changes are kept simple,

    only tackling CConnman* and BanMan* pointers
    only within the net_processing module, i.e. no changes that would
need adaption in other modules
    keeping the names of the variables as they are

ninja all check-all