Faster Processing of Master Archive Index

Recently, I revisited the code for the master archive index plugin, master_index.sh, and found out why it's so slow. Basically, out of laziness, the entire entry's content was being loaded, using the read_entry() function. The problem, is that there's no easy way to get a quick listing of an entry's related categories (yet), so the code was indirectly relying on the category links plugin. To reduce the unnecessary overhead, the code addresses the category links plugin directly and replaces the use of the read_entry function with the much quicker, read_metadata function. The results seem to indicate a speed increase of approximately 50 percent!