For a Hypergrid user, delay estate access checks until NewUserConnection() so that they work.

This is necessary because the hypergrid groups checks (as referenced by estates) require an agent circuit to be present to construct the hypergrid ID.
However, this is not around until Scene.NewUserConnection(), as called by CreateAgent() in EntityTransferModule.
Therefore, if we're dealing with a hypergrid user, delay the check until NewUserConnection()/CreateAgent()
The entity transfer impact should be minimal since CreateAgent() is the next significant call after NewUserConnection()
However, to preserve the accuracy of query access we will only relax the check for HG users.
Also check user authorization if looking to upgrade from a child to a root agent.

Relevant if a child agent has been allowed into the region which should not be upgraded to a root agent.
Stop "show client stats" from throwing an exception if somehow Scene.m_clientManager still retains a reference to a dead client.

Instead, "show client stats" now prints "Off!" so that exception is not thrown and we know which entries in ClientManager are in this state.
There's a race condition which could trigger this, but the window is extremely short and exceptions would not be thrown consistently (which is the behaviour observed).
It should otherwise be impossible for this condition to occur, so there may be a weakness in client manager IClientAPI removal.
minor: add doc about DefaultHGRegion and some of the other GridService region settings (though not all as of yet)