Server class code cleanups (#9769)

* Server::overrideDayNightRatio doesn't require to return bool
There is no sense to sending null player, the caller should send a valid object

* Server::init: make private & cleanup
This function is always called before start() and loads some variables which can be loaded in constructor directly.
Make it private and call it directly in start

* Split Server inventory responsibility to a dedicated object

This splits permit to found various historical issues:
* duplicate lookups on player connection
* sending inventory to non related player when a player connects
* non friendly lookups on detached inventories ownership

This reduce the detached inventory complexity and also increased the
lookup performance in a quite interesting way for servers with thousands
of inventories.

src/script/lua_api/l_base.cpp

@@ -45,6 +45,11 @@ Server *ModApiBase::getServer(lua_State *L)
 	return getScriptApiBase(L)->getServer();
 }
 
+ServerInventoryManager *ModApiBase::getServerInventoryMgr(lua_State *L)
+{
+	return getScriptApiBase(L)->getServer()->getInventoryMgr();
+}
+
 #ifndef SERVER
 Client *ModApiBase::getClient(lua_State *L)
 {