chatroomread.php doesn't avoid bootstraps when there are no messages in chat

The Problem:

In chatroomread.php we have:

if (!$skip_cache && file_exists($chat_cache_file)) { .. }

And the $chat_cache_file file doesn't exist when there are no messages in that chat.
This leads to costly and unnecessary Drupal bootstraps.

The Solution:
Create a chat cache file on hook_insert (and delete it on hook_delete).
- see attached patch.