Now, moving a value to the head of the queue is O(1). Before it was

Now, moving a value to the head of the queue is O(1).  Before it was
O(queue size).  This improves performance of individual flushes when
there is a large number of files in the queue.  As a result, we don't
hold the cache_lock as much.

Revamped enqueue_cache_item to take advantage of the new structure.

Renamed _wipe_ci_values to look nicer with other code.

--kevin

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk@1555 a5681a0c-68f1-0310-ab6d-d61299d08faa