[Patch] DB_common dealing with prepared statements

I noticed that there wasn't any checking taking place during prepare(), this patch checks the previously prepared statements (if any) and does not bother to prepare a duplicate statement by checking prepared_queries for the current query being prepared. This may not be a huge issue for most, but here is where I noticed it: I have an object (AudioCD) that contains a reference to a DB object. The AudioCD has a method, getTracks() that executes some SQL to get a list of tracks and then instantiates a new Track object for each one, passing in a reference to the DB object and the primary key of the object to create. After instantiating all of the tracks for a given CD I noticed that prepare_tokens had many duplicate entries, due to the SQL being prepared for each Track all on the same DB object. Michael