Move the inventory request lock introduced in git master a58152bd to HGInventoryBroker to preserve that behaviour there but allow 2 simultaneous inv requests (chiefly WebFetch) rather than 1

This lock serialized all requests and made the inventory throttling in WebFetch redundant.
By moving this lock, two simultaneous requests may now take place which may help with http://opensimulator.org/mantis/view.php?id=7054