Aims to address http://opensimulator.org/mantis/view.php?id=6355
As part of this work, an incomplete IXGroupsData was added which currently only allows store/fetch/delete of group records
(i.e. no membership data etc)
This is subject to change and currently only an in-memory storage implementation exists for regression test purposes.
Merged ArchiveWriteRequestPreparation.cs and ArchiveWriteRequestExecution.cs. This simplifies the code, and it's faster to write each scene to the archive as it's found rather than all at once at the end.
When this switch is used, iar folders are merged with existing same-name user inventory folders.
This makes it a little easier to back and restore entire individual user inventories, among other things
Added unit test to check behaviour
