Fix the presence info caching used in llRequestAgentData(), which was completely inoperative.

This means the presence info may be out of date by up to 20 seconds, but this avoids scripts potentially triggering constants requests to user accout and presence info services.
Relates to http://opensimulator.org/mantis/view.php?id=7088 though I fixed in a different way.
Adds regression test for this case.