Move areas:save() into async

On newer Minetest servers, handles saving jobs in async environment. To prevent conflicts, the save file is locked whie saving, and if a code requests saving while the file is locked, data is saved again immediately after finishing the current save.

async.lua

@@ -0,0 +1,23 @@
+areas = rawget(_G, "areas") or {}
+
+local safe_file_write = core.safe_file_write
+if safe_file_write == nil then
+    safe_file_write = function(path, content)
+        local file, err = io.open(path, "w")
+        if err then
+            return err
+        end
+        file:write(content)
+        file:close()
+    end
+end
+
+-- Save the areas table to a file
+function areas._internal_do_save(areas_tb, filename)
+    local datastr = core.write_json(areas_tb)
+    if not datastr then
+        core.log("error", "[areas] Failed to serialize area data!")
+        return
+    end
+    return safe_file_write(filename, datastr)
+end