campaignd: Replace vector<pair<k,v>> with map<k,v> for the handlers table

std::vector<std::pair<k,v>> has worse look-up performance than
std::map<k,v> in general, not that the difference is too noticeable for
such a small dataset like campaignd's handlers table, that is built in
such a way its items are arranged in descending order of usage
frequency.

It still feels like this is the best thing to do to keep future
maintainers sane.