[SON-HIVE Voting] Refactor all the places we use sidechain_type directly

There are places in code where we call sidechain_type::bitcoin or sidechain_type::hive directly.

Like this:

flat_map<sidechain_type, vector<std::reference_wrapper<const son_object> > > sons;
sons[sidechain_type::bitcoin] = sort_votable_objects<son_index>(sidechain_type::bitcoin, cp.maximum_son_count());
sons[sidechain_type::hive] = sort_votable_objects<son_index>(sidechain_type::hive, cp.maximum_son_count());

Need to refactor those places to use a more generic way. Like this:

for(const auto& active_sidechain_type : active_sidechain_types)
{
  sons[active_sidechain_type] = sort_votable_objects<son_index>(active_sidechain_type, cp.maximum_son_count());
}