If this is allowed, then the client usually gets forcibly logged out and data structures might be put into bad states.
To prevent this, the binary state machine of EMT.m_agentsInTransit is replaced with a 4 state machine (Preparing, Transferring, ReceivedAtDestination, CleaningUp).
This is necessary because the source region needs to know when the destination region has received the user but a teleport back cannot happen until the source region has cleaned up.
Tested on standalone, grid and with v1 and v3 clients.
This is to help relieve a race condition when an agent teleports then immediately attempts to teleport back before the source region has properly cleaned up/demoted the old ScenePresence.
This is rare in viewers but much more possible via scripting or region module.
However, more needs to be done since virtually all clean up happens after the transit flag is cleared .
Possibly need to add a 'cleaning up' state to in transit.
This change required making the EntityTransferModule and HGEntityTransferModule per-region rather than shared, in order to allow separate transit lists.
Changes were also required in LocalSimulationConnector.
Tested in standalone, grid and with local and remote region crossings with attachments.
