Add post-CreateAgent teleport cancellation/abortion functionality from v1 transfer protocol into v2.

This stops OpenSimulator still trying to teleport the user if they hit cancel on the teleport screen or closed the viewer whilst the protocol was trying to create an agent on the remote region.
Ideally, the code may also attempt to tell the destination simulator that the agent should be removed (accounting for issues where the destination was not responding in the first place, etc.)