* empty function
* default constructor/destructor
* for range-based loops
* use emplace_back instead of push_back
* remove some unused headers in some cpp variable
* mod can create a ModMetadata object where store its values and retrieve it.
* Modmetadata object can only be fetched at mod loading
* Save when modified using same time as map interval or at server stop
* add helper function to get mod storage path
* ModMetadata has exactly same calls than all every other Metadata
