Fix bug where crossing to a neighbouring region and back again would trigger an exception, and a second recross would stop the user moving until relog

Also fixes an issue where sitting avatar counts became inaccurate after any cross.
Part of the problem was due to cloning code using MemberwiseClone() but not resetting certain collection structures.
Adds regression test for this case.
In relation to http://opensimulator.org/mantis/view.php?id=7050