refactor: replace CConnman pointers by references in net_processing.cpp
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
Backport of core PR19174.
Depends on D8479.
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8480