Allow one to specify a DefaultHGRegion flag in [GridService] in order to allow different default regions for HG and direct grid logins.

This requires a new GridService.GetDefaultHypergridRegions() so ROBUST services require updating but not simulators.
This method still returns regions flagged with just DefaultRegion after any DefaultHGRegions, so if no DefaultHGRegions are specified
then existing configured defaults will still work.
Immediate use is for conference where we need to be able to specify different defaults
However, this is also generally useful to send experienced HG users to one default location and local users whose specified region fails (e.g. no "home" or "last") to another.
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.