Refactor query string processing

[mmd-osm]

Query string parsing is a bit of a mess right now. Parsing should be done only once, and results stored in an std::map in "struct request". This needs to work both for fcgi, as well as unit tests.

Example 1:

  string decoded = http::urldecode(get_query_string(req));
  const auto params = http::parse_params(decoded);
  auto itr =
    std::find_if(params.begin(), params.end(),
        [](const pair<string, string> &p) { return p.first == "bbox"; });
        

Example 2:

std::vector<id_version> parse_id_list_params(const request &req,
                                             std::string_view param_name) {

  std::string decoded = http::urldecode(get_query_string(req));
  const auto params = http::parse_params(decoded);

  auto itr = std::find_if(params.begin(), params.end(), [&param_name](auto &x) {
    return x.first == param_name;
  });

  if (itr == params.end())
    return {};

  const std::string &str = itr->second;
            

(there are a few others as well).