Instead of auto-creating a parcel on request if one doesn't cover a given location, fill in gaps or extend existing parcel right after initial data load.

This is in line with simpler and still existing behaviour where a default parcel is created if no parcels are in storage at all.
Making this change as another step to address current problems with spurious parcels occasionally being created.
Also adds regression tests for different load behaviours depending on existing stored parcel data.
Relates to http://opensimulator.org/mantis/view.php?id=7035
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim