* CacheMap::Insert should not update existing item, should return `false`
* Use `emplace` operator instead of `[]` to add new items in Cache*Map implementation
* Use prefix cm/cmm for CacheMap/CacheMultiMap maps respectively to distinguish them from each other and from std::map-s
* Avoid excessive processing of already known valid votes
Also prettify debug log output a bit
* Drop nCurrentSize from Cache*Map classes
Use `size()` and `empty()` of std classes instead, they have constant complexity since c++11
* Do not create an explicit iterator if it's never used later (in Cache*Map)
* Do not prune last item in CacheMultiMapInsert when trying to insert a duplicate one
