Comment out warning about agent updating without valid session ID for now.

This causes extreme console spam if a simulator running latest master and one running 0.7.5 have adjacent regions occupied by avatars.
Fix bug where users teleporting to non-neighbour regions could continue to hear chat from their source region for some time after teleport completion.

This occurs on v2 teleport since the source region now waits 15 secs before closing the old child agent, which could still receive chat.
This commit introduces a ScenePresenceState.PreClose which is set before the wait, so that ChatModule can check for ScenePresenceState.Running.
This was theoretically also an issue on v1 teleport but since the pause before close was only 2 secs there, it was not noticed.